Wounded passengers are treated following a suicide bombing at the Brussels Airport on March 22, 2016. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2016%2F03%2F30%2Feurope%2Fbrussels-investigation%2Findex.html">The attacks on the airport and a subway </a>killed 32 people and wounded more than 300. ISIS claims its "fighters" launched the attacks in the Belgian capital.
Wounded passengers are treated following a suicide bombing at the Brussels Airport on March 22, 2016. The attacks on the airport and a subway killed 32 people and wounded more than 300. ISIS claims its "fighters" launched the attacks in the Belgian capital.
Ketevan Kardava/Getty Images
Syrians gather at the site of a double car bomb attack in the Al-Zahraa neighborhood of the Homs, Syria, on February 21, 2016. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2016%2F02%2F21%2Fmiddleeast%2Fsyria-civil-war%2Findex.html" target="_blank">Multiple attacks in Homs and southern Damascus</a> kill at least 122 and injure scores, according to the state-run SANA news agency. ISIS claimed responsibility.
Syrians gather at the site of a double car bomb attack in the Al-Zahraa neighborhood of the Homs, Syria, on February 21, 2016. Multiple attacks in Homs and southern Damascus kill at least 122 and injure scores, according to the state-run SANA news agency. ISIS claimed responsibility.
STRINGER/AFP/Getty Images
Syrian pro-government forces gather at the site of a deadly triple bombing Sunday, January 31, in the <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2016%2F01%2F31%2Fmiddleeast%2Fsyria-damascus-explosions%2Findex.html" target="_blank">Damascus suburb of Sayeda Zeynab</a>. ISIS claimed responsibility for the attack, according to a statement circulating online from supporters of the terrorist group.
Syrian pro-government forces gather at the site of a deadly triple bombing Sunday, January 31, in the Damascus suburb of Sayeda Zeynab. ISIS claimed responsibility for the attack, according to a statement circulating online from supporters of the terrorist group.
LOUAI BESHARA/AFP/Getty Images
Yemenis check the scene of a car bomb attack Sunday, December 6, in Aden, Yemen. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F12%2F06%2Fmiddleeast%2Fyemen-aden-governor-killed%2Findex.html">Aden Gov. Jaafar Saad and six bodyguards died in the attack</a>, for which the terror group ISIS claimed responsibility.
Yemenis check the scene of a car bomb attack Sunday, December 6, in Aden, Yemen. Aden Gov. Jaafar Saad and six bodyguards died in the attack, for which the terror group ISIS claimed responsibility.
Wael Qubady/AP
Investigators check the scene of a mosque attack Friday, November 27, in northern Bangladesh's Bogra district. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F11%2F27%2Fasia%2Fbangladesh-isis-attack-claim%2Findex.html" target="_blank">ISIS has claimed responsibility for the attack</a> that left at least one person dead and three more wounded.
Investigators check the scene of a mosque attack Friday, November 27, in northern Bangladesh's Bogra district. ISIS has claimed responsibility for the attack that left at least one person dead and three more wounded.
AP
Wounded people are helped outside the Bataclan concert hall in Paris following a series of coordinated attacks in the city on Friday, November 13. The militant group ISIS claimed responsibility <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F11%2F13%2Fworld%2Fgallery%2Fparis-attacks%2Findex.html" target="_blank">for the attacks,</a> which killed at least 130 people and wounded hundreds more.
Wounded people are helped outside the Bataclan concert hall in Paris following a series of coordinated attacks in the city on Friday, November 13. The militant group ISIS claimed responsibility for the attacks, which killed at least 130 people and wounded hundreds more.
YOAN VALAT/EPA/LANDOV
Emergency personnel and civilians gather at the site of a <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F11%2F16%2Fmiddleeast%2Fbeirut-explosions%2F" target="_blank">twin suicide bombing</a> in Beirut, Lebanon, on Thursday, November 12. The bombings killed at least 43 people and wounded more than 200 more. ISIS appeared to claim responsibility in a statement posted on social media.
Emergency personnel and civilians gather at the site of a twin suicide bombing in Beirut, Lebanon, on Thursday, November 12. The bombings killed at least 43 people and wounded more than 200 more. ISIS appeared to claim responsibility in a statement posted on social media.
Bilal Hussein/AP
Smoke rises over the northern Iraqi town of Sinjar on November 12. Kurdish Iraqi fighters, backed by a U.S.-led air campaign, <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F11%2F13%2Fmiddleeast%2Firaq-free-sinjar-isis%2F" target="_blank">retook the strategic town, </a>which ISIS militants overran last year. ISIS wants to create an Islamic state across Sunni areas of Iraq and Syria.
Smoke rises over the northern Iraqi town of Sinjar on November 12. Kurdish Iraqi fighters, backed by a U.S.-led air campaign, retook the strategic town, which ISIS militants overran last year. ISIS wants to create an Islamic state across Sunni areas of Iraq and Syria.
Bram Janssen/AP
Syrian government troops walk inside the Kweiras air base on Wednesday, November 11, after they broke a siege imposed by ISIS militants.
Syrian government troops walk inside the Kweiras air base on Wednesday, November 11, after they broke a siege imposed by ISIS militants.
SANA/AP
Members of the Egyptian military approach the wreckage of a Russian passenger plane Sunday, November 1, in Hassana, Egypt. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F10%2F31%2Fworld%2Fgallery%2Frussian-plane-crash%2Findex.html" target="_blank">The plane crashed</a> the day before, killing all 224 people on board. ISIS claimed responsibility for downing the plane, but the group's claim wasn't immediately verified.
Members of the Egyptian military approach the wreckage of a Russian passenger plane Sunday, November 1, in Hassana, Egypt. The plane crashed the day before, killing all 224 people on board. ISIS claimed responsibility for downing the plane, but the group's claim wasn't immediately verified.
Maxim Grigoriev/Russian Ministry for Emergency Situations/AP
An explosion rocks Kobani, Syria, during a reported car bomb attack by ISIS militants on Tuesday, October 20.
An explosion rocks Kobani, Syria, during a reported car bomb attack by ISIS militants on Tuesday, October 20.
Gokhan Sahin/Getty Images
Shiite fighters, fighting alongside Iraqi government forces, fire a rocket at ISIS militants as they advance toward the center of Baiji, Iraq, on Monday, October 19.
Shiite fighters, fighting alongside Iraqi government forces, fire a rocket at ISIS militants as they advance toward the center of Baiji, Iraq, on Monday, October 19.
AHMAD AL-RUBAYE/AFP/Getty Images
Smoke rises above a damaged building in Ramadi, Iraq, following a coalition airstrike against ISIS positions on Saturday, August 15.
Smoke rises above a damaged building in Ramadi, Iraq, following a coalition airstrike against ISIS positions on Saturday, August 15.
AP
Iraqi men look at damage following a bomb explosion that targeted a vegetable market in Baghdad on Thursday, August 13. ISIS claimed responsibility for the attack.
Iraqi men look at damage following a bomb explosion that targeted a vegetable market in Baghdad on Thursday, August 13. ISIS claimed responsibility for the attack.
AHMAD AL-RUBAYE/AFP/AFP/Getty Images
In this image taken from social media, an ISIS fighter holds the group's flag after the militant group <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F08%2F07%2Fworld%2Fsyria-isis-al-qaryatayn-christians%2Findex.html" target="_blank">overran the Syrian town of al-Qaryatayn</a> on Thursday, August 6, the London-based Syrian Observatory for Human Rights reported. ISIS uses modern tools such as social media to promote reactionary politics and religious fundamentalism. Fighters are destroying holy sites and valuable antiquities even as their leaders propagate a return to the early days of Islam.
In this image taken from social media, an ISIS fighter holds the group's flag after the militant group overran the Syrian town of al-Qaryatayn on Thursday, August 6, the London-based Syrian Observatory for Human Rights reported. ISIS uses modern tools such as social media to promote reactionary politics and religious fundamentalism. Fighters are destroying holy sites and valuable antiquities even as their leaders propagate a return to the early days of Islam.
From ISIS
An ISIS fighter poses with spoils purportedly taken after capturing the Syrian town of al-Qaryatayn.
An ISIS fighter poses with spoils purportedly taken after capturing the Syrian town of al-Qaryatayn.
From ISIS
Smoke rises as Iraqi security forces bomb ISIS positions in the eastern suburbs of Ramadi, Iraq, on August 6.
Smoke rises as Iraqi security forces bomb ISIS positions in the eastern suburbs of Ramadi, Iraq, on August 6.
AP
Buildings reduced to piles of debris can be seen in the eastern suburbs of Ramadi on August 6.
Buildings reduced to piles of debris can be seen in the eastern suburbs of Ramadi on August 6.
AP
The governor of the Asir region in Saudi Arabia, Prince Faisal bin Khaled bin Abdulaziz, left, visits a man who was wounded in <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F08%2F06%2Fmiddleeast%2Fsaudi-arabia-mosque-attack%2F" target="_blank">a suicide bombing attack on a mosque</a> in Abha, Saudi Arabia, on August 6. ISIS claimed responsibility for the explosion, which killed at least 13 people and injured nine others.
The governor of the Asir region in Saudi Arabia, Prince Faisal bin Khaled bin Abdulaziz, left, visits a man who was wounded in a suicide bombing attack on a mosque in Abha, Saudi Arabia, on August 6. ISIS claimed responsibility for the explosion, which killed at least 13 people and injured nine others.
Saudi Press Agency/AP
Saudi officials and investigators check the inside of the mosque on August 6.
Saudi officials and investigators check the inside of the mosque on August 6.
Saudi Press Agency/AP
Mourners in Gaziantep, Turkey, grieve over a coffin Tuesday, July 21, during a funeral ceremony for the victims of a suspected ISIS suicide bomb attack. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F07%2F20%2Fworld%2Fturkey-suruc-explosion%2F">That bombing killed at least 31 people</a> in Suruc, a Turkish town that borders Syria. Turkish authorities blamed ISIS for the attack.
Mourners in Gaziantep, Turkey, grieve over a coffin Tuesday, July 21, during a funeral ceremony for the victims of a suspected ISIS suicide bomb attack. That bombing killed at least 31 people in Suruc, a Turkish town that borders Syria. Turkish authorities blamed ISIS for the attack.
Gokhan Sahin/Getty Images
Protesters in Istanbul carry anti-ISIS banners and flags to show support for victims of the Suruc suicide blast during a demonstration on Monday, July 20.
Protesters in Istanbul carry anti-ISIS banners and flags to show support for victims of the Suruc suicide blast during a demonstration on Monday, July 20.
YASIN AKGUL/AFP/Getty Images
People in Ashmoun, Egypt, carry the coffin for 1st Lt. Mohammed Ashraf, who was killed when the ISIS militant group <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F07%2F02%2Fworld%2Fisis-egypt-expanding-reach%2Findex.html" target="_blank">attacked Egyptian military checkpoints</a> on Wednesday, July 1. At least 17 soldiers were reportedly killed, and 30 were injured.
People in Ashmoun, Egypt, carry the coffin for 1st Lt. Mohammed Ashraf, who was killed when the ISIS militant group attacked Egyptian military checkpoints on Wednesday, July 1. At least 17 soldiers were reportedly killed, and 30 were injured.
Ashour Abosalm/AP
Syrians wait near the Turkish border during clashes between ISIS and Kurdish armed groups in Kobani, Syria, on Thursday, June 25. The photo was taken in Sanliurfa, Turkey. ISIS militants disguised as Kurdish security forces infiltrated Kobani on Thursday and killed "many civilians," said a spokesman for the Kurds in Kobani.
Syrians wait near the Turkish border during clashes between ISIS and Kurdish armed groups in Kobani, Syria, on Thursday, June 25. The photo was taken in Sanliurfa, Turkey. ISIS militants disguised as Kurdish security forces infiltrated Kobani on Thursday and killed "many civilians," said a spokesman for the Kurds in Kobani.
Halil Fidan/Anadolu Agency/Getty Images
Residents examine a damaged mosque after an Iraqi Air Force bombing in the ISIS-seized city of Falluja, Iraq, on Sunday, May 31. At least six were killed and nine others wounded during the bombing.
Residents examine a damaged mosque after an Iraqi Air Force bombing in the ISIS-seized city of Falluja, Iraq, on Sunday, May 31. At least six were killed and nine others wounded during the bombing.
Sami Jawad/Xinhua/SIPA
People search through debris after an explosion at a Shiite mosque in Qatif, Saudi Arabia, on Friday, May 22. ISIS <a href="index.php?page=&url=http%3A%2F%2Fedition.cnn.com%2F2015%2F05%2F22%2Fmiddleeast%2Fsaudi-arabia-mosque-blast%2Findex.html" target="_blank">claimed responsibility for the attack,</a> according to tweets from ISIS supporters, which included a formal statement from ISIS detailing the operation.
People search through debris after an explosion at a Shiite mosque in Qatif, Saudi Arabia, on Friday, May 22. ISIS claimed responsibility for the attack, according to tweets from ISIS supporters, which included a formal statement from ISIS detailing the operation.
EPA/STR/LANDOV
Iraqi soldiers fire their weapons toward ISIS group positions in the Garma district, west of the Iraqi capital of Baghdad, on Sunday, April 26. Pro-government forces said they had recently made advances on areas held by Islamist jihadists.
Iraqi soldiers fire their weapons toward ISIS group positions in the Garma district, west of the Iraqi capital of Baghdad, on Sunday, April 26. Pro-government forces said they had recently made advances on areas held by Islamist jihadists.
AHMAD AL-RUBAYE/AFP/Getty Images
A member of Afghanistan's security forces stands at the site where a suicide bomber on a motorbike blew himself up in front of the Kabul Bank in Jalalabad, Afghanistan, on Saturday, April 18. ISIS claimed responsibility for the attack. The explosion killed at least 33 people and injured more than 100 others, a public health spokesman said.
A member of Afghanistan's security forces stands at the site where a suicide bomber on a motorbike blew himself up in front of the Kabul Bank in Jalalabad, Afghanistan, on Saturday, April 18. ISIS claimed responsibility for the attack. The explosion killed at least 33 people and injured more than 100 others, a public health spokesman said.
Stringer/AP
Iraqi counterterrorism forces patrol in Ramadi on April 18.
Iraqi counterterrorism forces patrol in Ramadi on April 18.
Stringer/AP
Thousands of Iraqis cross a bridge over the Euphrates River to Baghdad as they flee Ramadi on Friday, April 17.
Thousands of Iraqis cross a bridge over the Euphrates River to Baghdad as they flee Ramadi on Friday, April 17.
Ali Mohammed/Anadolu Agency/Getty Images
Yazidis embrace after being released by ISIS south of Kirkuk, Iraq, on Wednesday, April 8.<a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F04%2F08%2Fworld%2Fisis-yazidis-released%2F"> ISIS released more than 200 Yazidis</a>, a minority group whose members were killed, captured and displaced when the Islamist terror organization overtook their towns in northern Iraq last summer, officials said.
Yazidis embrace after being released by ISIS south of Kirkuk, Iraq, on Wednesday, April 8. ISIS released more than 200 Yazidis, a minority group whose members were killed, captured and displaced when the Islamist terror organization overtook their towns in northern Iraq last summer, officials said.
Ali Mukarrem Garip/Anadolu Agency/Getty Images
Kurdish Peshmerga forces help Yazidis as they arrive at a medical center in Altun Kupri, Iraq, on April 8.
Kurdish Peshmerga forces help Yazidis as they arrive at a medical center in Altun Kupri, Iraq, on April 8.
SAFIN HAMED/AFP/Getty Images
A Yazidi woman mourns for the death of her husband and children by ISIS after being released south of Kirkuk on April 8. ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts.
A Yazidi woman mourns for the death of her husband and children by ISIS after being released south of Kirkuk on April 8. ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts.
Feriq Ferec/Anadolu Agency/Getty Images
People in Tikrit inspect what used to be a palace of former President Saddam Hussein on April 3.
People in Tikrit inspect what used to be a palace of former President Saddam Hussein on April 3.
Khalid Mohammed/AP
On April 1, Shiite militiamen celebrate the retaking of Tikrit, which had been under ISIS control since June. The push into Tikrit came days after U.S.-led airstrikes targeted ISIS bases around the city.
On April 1, Shiite militiamen celebrate the retaking of Tikrit, which had been under ISIS control since June. The push into Tikrit came days after U.S.-led airstrikes targeted ISIS bases around the city.
AHMAD AL-RUBAYE/AFP/Getty Images
Iraqi security forces launch a rocket against ISIS positions in Tikrit on Monday, March 30.
Iraqi security forces launch a rocket against ISIS positions in Tikrit on Monday, March 30.
Khalid Mohammed/AP
The parents of 19-year-old Mohammed Musallam react at the family's home in the East Jerusalem Jewish settlement of Neve Yaakov on Tuesday, March 10. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F03%2F10%2Fmiddleeast%2Fisis-video-israeli-killed%2F">ISIS released a video purportedly</a> showing a young boy executing Musallam, an Israeli citizen of Palestinian descent who ISIS claimed infiltrated the group in Syria to spy for the Jewish state. Musallam's family told CNN that he had no ties with the Mossad, Israel's spy agency, and had, in fact, been recruited by ISIS.
The parents of 19-year-old Mohammed Musallam react at the family's home in the East Jerusalem Jewish settlement of Neve Yaakov on Tuesday, March 10. ISIS released a video purportedly showing a young boy executing Musallam, an Israeli citizen of Palestinian descent who ISIS claimed infiltrated the group in Syria to spy for the Jewish state. Musallam's family told CNN that he had no ties with the Mossad, Israel's spy agency, and had, in fact, been recruited by ISIS.
Ahmed Gharabli/AFP/Getty Images
Iraqi Shiite fighters cover their ears as a rocket is launched during a clash with ISIS militants in the town of Al-Alam, Iraq, on Monday, March 9.
Iraqi Shiite fighters cover their ears as a rocket is launched during a clash with ISIS militants in the town of Al-Alam, Iraq, on Monday, March 9.
Thaier Al-Sudani/Reuters
Displaced Assyrian women who fled their homes due to ISIS attacks pray at a church on the outskirts of Damascus, Syria, on Sunday, March 1. ISIS militants abducted at least 220 Assyrians in Syria.
Displaced Assyrian women who fled their homes due to ISIS attacks pray at a church on the outskirts of Damascus, Syria, on Sunday, March 1. ISIS militants abducted at least 220 Assyrians in Syria.
LOUAI BESHARA/AFP/Getty Images
Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on Wednesday, February 4. Al-Kasasbeh's son, <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2F2015%2F02%2F03%2Fworld%2Fgallery%2Fjordanian-pilot-reaction%2Findex.html" target="_blank">Jordanian pilot Moath al-Kasasbeh,</a> was burned alive in a video that was recently released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.
Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on Wednesday, February 4. Al-Kasasbeh's son, Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was recently released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.
Nasser Nasser/AP
A Kurdish marksman looks over a destroyed area of Kobani on Friday, January 30, after the city had been liberated from the ISIS militant group. The Syrian city, also known as Ayn al-Arab, had been under assault by ISIS since mid-September.
A Kurdish marksman looks over a destroyed area of Kobani on Friday, January 30, after the city had been liberated from the ISIS militant group. The Syrian city, also known as Ayn al-Arab, had been under assault by ISIS since mid-September.
BULENT KILIC/AFP/Getty Images
Kurdish people celebrate in Suruc, Turkey, near the Turkish-Syrian border, after ISIS militants were expelled from Kobani on Tuesday, January 27.
Kurdish people celebrate in Suruc, Turkey, near the Turkish-Syrian border, after ISIS militants were expelled from Kobani on Tuesday, January 27.
BULENT KILIC/AFP/Getty Images
Collapsed buildings are seen in Kobani on January 27 after Kurdish forces took control of the town from ISIS.
Collapsed buildings are seen in Kobani on January 27 after Kurdish forces took control of the town from ISIS.
Rauf Maltas/Anadolu Agency/Getty Images
Junko Ishido, mother of Japanese journalist Kenji Goto, reacts during a news conference in Tokyo on Friday, January 23. ISIS would later kill Goto and another Japanese hostage, Haruna Yukawa.
Junko Ishido, mother of Japanese journalist Kenji Goto, reacts during a news conference in Tokyo on Friday, January 23. ISIS would later kill Goto and another Japanese hostage, Haruna Yukawa.
Toru Hanai/Reuters/LANDOV
ISIS militants are seen through a rifle's scope during clashes with Peshmerga fighters in Mosul, Iraq, on Wednesday, January 21.
ISIS militants are seen through a rifle's scope during clashes with Peshmerga fighters in Mosul, Iraq, on Wednesday, January 21.
Emrah Yorulmaz/Anadolu Agency/Getty Images
An elderly Yazidi man arrives in Kirkuk after being released by ISIS on Saturday, January 17. The militant group released about 200 Yazidis who were held captive for five months in Iraq. Almost all of the freed prisoners were in poor health and bore signs of abuse and neglect, Kurdish officials said.
An elderly Yazidi man arrives in Kirkuk after being released by ISIS on Saturday, January 17. The militant group released about 200 Yazidis who were held captive for five months in Iraq. Almost all of the freed prisoners were in poor health and bore signs of abuse and neglect, Kurdish officials said.
AP
Smoke billows behind an ISIS sign during an Iraqi military operation to regain control of the town of Sadiyah, about 95 kilometers (60 miles) north of Baghdad, on Tuesday, November 25.
Smoke billows behind an ISIS sign during an Iraqi military operation to regain control of the town of Sadiyah, about 95 kilometers (60 miles) north of Baghdad, on Tuesday, November 25.
AP
Fighters from the Free Syrian Army and the Kurdish People's Protection Units join forces to fight ISIS in Kobani on Wednesday, November 19.
Fighters from the Free Syrian Army and the Kurdish People's Protection Units join forces to fight ISIS in Kobani on Wednesday, November 19.
Jake Simkin/AP
A picture taken from Turkey shows smoke rising after ISIS militants fired mortar shells toward an area controlled by Syrian Kurdish fighters near Kobani on Monday, November 3.
A picture taken from Turkey shows smoke rising after ISIS militants fired mortar shells toward an area controlled by Syrian Kurdish fighters near Kobani on Monday, November 3.
ULAS YUNUS TOSUN/EPA/Landov
Iraqi special forces search a house in Jurf al-Sakhar, Iraq, on Thursday, October 30, after retaking the area from ISIS.
Iraqi special forces search a house in Jurf al-Sakhar, Iraq, on Thursday, October 30, after retaking the area from ISIS.
HAIDAR HAMDANI/AFP/Getty Images
ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23. The United States and several Arab nations have been bombing ISIS targets in Syria to take out the militant group's ability to command, train and resupply its fighters.
ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23. The United States and several Arab nations have been bombing ISIS targets in Syria to take out the militant group's ability to command, train and resupply its fighters.
BULENT KILIC/AFP/Getty Images
Kurdish fighters walk to positions as they combat ISIS forces in Kobani on Sunday, October 19.
Kurdish fighters walk to positions as they combat ISIS forces in Kobani on Sunday, October 19.
Gokhan Sahin/Getty Images
Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on October 18.
Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on October 18.
Gokhan Sahin/Getty Images
Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.
Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.
Gokhan Sahin/Getty Images
Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.
Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.
Lefteris Pitarakis/ap
Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.
Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.
Aris Messinis/AFP/Getty Images
A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk, Iraq, on Tuesday, September 30.
A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk, Iraq, on Tuesday, September 30.
Hadi Mizban/AP
Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.
Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.
MURAD SEZER/Reuters/Landov
A elderly man is carried after crossing the Syria-Turkey border near Suruc on Saturday, September 20.
A elderly man is carried after crossing the Syria-Turkey border near Suruc on Saturday, September 20.
BULENT KILIC/AFP/Getty Images
A Kurdish Peshmerga fighter launches mortar shells toward ISIS militants in Zumar, Iraq, on Monday, September 15.
A Kurdish Peshmerga fighter launches mortar shells toward ISIS militants in Zumar, Iraq, on Monday, September 15.
AHMED JADALLAH/Reuters/Landov
Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.
Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.
JM LOPEZ/AFP/Getty Images
Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.
Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.
Khalid Mohammed/AP
Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.
Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.
Warzer Jaff/CNN
Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.
Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.
Emrah Yorulmaz/Anadolu Agency/Getty Images
Thousands of Yazidi and Christian people flee Mosul on Wednesday, August 6, after the latest wave of ISIS advances.
Thousands of Yazidi and Christian people flee Mosul on Wednesday, August 6, after the latest wave of ISIS advances.
Mustafa Kerim/Anadolu Agency/Getty Images
A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.
A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.
Stringer/Anadolu Agency/Getty mages
A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.
A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.
ABD DOUMANY/AFP/Getty Images
Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.
Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.
STRINGER/IRAQ/reuters/LANDOV
The ISIS terror threat
CNN  — 

ISIS militants have retaken the key Syrian city of Ain Issa from Kurdish forces, the London-based Syrian Observatory for Human Rights said Monday.

Backed by U.S.-led coalition airstrikes, Kurdish forces wrested control of Ain Issa and Tal Abyad last month.

Both cities are strategically important, because they sit along a supply route on a main road between Raqqa, the city ISIS considers to be its capital, and the Turkish border.

The development comes after a weekend of coalition airstrikes on ISIS targets in Raqqa, described by Brett McGurk, the U.S. envoy for the coalition against ISIS, as “the most sustained set of airstrikes to date” against ISIS in Syria.

CNN’s Gul Tuysuz in Istanbul and Raja Razek in Beirut contributed to this report.