Tensions between Ukraine and Russia are high, with reports of a Russian troop build-up at the border fueling fears over Moscow's intentions.

Ukraine says Russia is trying to destabilize the country. Western powers have repeatedly warned Russia in recent weeks against any potential escalation. But the Kremlin denies it is planning an invasion, and argues that NATO support for Ukraine -- including increased weapons supplies and military training -- constitutes a growing threat on Russia's western flank.

Ukrainian soldiers march in formation during a military exercise at the International Center for Peacekeeping and Security of the National Academy of Land Forces, on January 28.
Ukrainian soldiers march in formation during a military exercise at the International Center for Peacekeeping and Security of the National Academy of Land Forces, on January 28.
Mykola Tys/SOPA Images/LightRocket/Getty Images
Ukrainian servicemen of the Mechanized Brigade drive vehicles at an undisclosed location in the Luhansk Region, Ukraine, on January 25.
Ukrainian servicemen of the Mechanized Brigade drive vehicles at an undisclosed location in the Luhansk Region, Ukraine, on January 25.
Wolfgang Schwan/Anadolu Agency/Getty Images
Ukrainian servicemen of the Mechanized Brigade service vehicles at an undisclosed location in the Luhansk Region, Ukraine, on January 25.
Ukrainian servicemen of the Mechanized Brigade service vehicles at an undisclosed location in the Luhansk Region, Ukraine, on January 25.
Wolfgang Schwan/Anadolu Agency/Getty Images
Ground crew unload weapons and other military hardware delivered by the United States at Boryspil Airport near Kyiv on January 25 in Boryspil, Ukraine.
Ground crew unload weapons and other military hardware delivered by the United States at Boryspil Airport near Kyiv on January 25 in Boryspil, Ukraine.
Sean Gallup/Getty Images
A Ukrainian serviceman of the 30th Army Brigade seen outside of Svitlodarsk, Ukraine, on January 23.
A Ukrainian serviceman of the 30th Army Brigade seen outside of Svitlodarsk, Ukraine, on January 23.
Wolfgang Schwan/Anadolu Agency/Getty Images
A convoy of Russian armored vehicles moves along a highway in Crimea on Tuesday January 18. Russia has concentrated an estimated 100,000 troops with tanks and other heavy weapons near Ukraine in what the West fears could be a prelude to an invasion.
A convoy of Russian armored vehicles moves along a highway in Crimea on Tuesday January 18. Russia has concentrated an estimated 100,000 troops with tanks and other heavy weapons near Ukraine in what the West fears could be a prelude to an invasion.
AP
Members of the Ukrainian military in a front line trench at Katerynivka, in the Luhansk Oblast province of eastern Ukraine on Tuesday January 18.
Members of the Ukrainian military in a front line trench at Katerynivka, in the Luhansk Oblast province of eastern Ukraine on Tuesday January 18.
Tyler Hicks/The New York Times/Redux
Servicemen take their position in a trench at the line of separation near Yasne village, south west of Donetsk in eastern Ukraine on Friday January 14.
Servicemen take their position in a trench at the line of separation near Yasne village, south west of Donetsk in eastern Ukraine on Friday January 14.
Alexei Alexandrov/AP
Russian troops take part in drills at the Kadamovskiy firing range in the Rostov region in southern Russia, Tuesday, 14 December.
Russian troops take part in drills at the Kadamovskiy firing range in the Rostov region in southern Russia, Tuesday, 14 December.
AP
US Deputy Secretary of State Wendy Sherman (L) and Russian deputy Foreign Minister Sergei Ryabkov (R) pose for pictures as they attend security talks on soaring tensions over Ukraine at the US Permanent Mission in Geneva on January 10.
US Deputy Secretary of State Wendy Sherman (L) and Russian deputy Foreign Minister Sergei Ryabkov (R) pose for pictures as they attend security talks on soaring tensions over Ukraine at the US Permanent Mission in Geneva on January 10.
Denis Balisbouse/AFP/Getty Images
A Ukrainian Military Forces servicemen strokes a cat in a trench at the front line with Russia-backed separatists near to Avdiivka, southeastern Ukraine, on January 9.
A Ukrainian Military Forces servicemen strokes a cat in a trench at the front line with Russia-backed separatists near to Avdiivka, southeastern Ukraine, on January 9.
Anatolli Stepanov/AFP/Getty Images
A Ukrainian soldier takes a rest near a fighting position on the line of separation from pro-Russian rebels in the Donetsk region of Ukraine on December 31, 2021.
A Ukrainian soldier takes a rest near a fighting position on the line of separation from pro-Russian rebels in the Donetsk region of Ukraine on December 31, 2021.
Andriy Dubchak/AP
President Joe Biden speaks with Russian President Vladimir Putin on the phone from his private residence in Wilmington, Delaware, on Thursday December 30.
President Joe Biden speaks with Russian President Vladimir Putin on the phone from his private residence in Wilmington, Delaware, on Thursday December 30.
Adam Schultz/The White House/AP
Ukrainian Territorial Defense Forces, the military reserve of the Ukrainian Armed Forces, holding wooden replicas of Kalashnikov rifles, take part in a military exercise near Kiev on December 25.
Ukrainian Territorial Defense Forces, the military reserve of the Ukrainian Armed Forces, holding wooden replicas of Kalashnikov rifles, take part in a military exercise near Kiev on December 25.
Sergei Supinksy/AFP/Getty Images
German Chancellor Olaf Scholz, center, attends a round table as Georgia's Prime Minister Irakli Garibashvili, left, greets Ireland's Prime Minister Micheal Martin, right, at an Eastern Partnership Summit in Brussels with the Russian military buildup on Ukraine's border as the main point of focus on Wednesday, December 15.
German Chancellor Olaf Scholz, center, attends a round table as Georgia's Prime Minister Irakli Garibashvili, left, greets Ireland's Prime Minister Micheal Martin, right, at an Eastern Partnership Summit in Brussels with the Russian military buildup on Ukraine's border as the main point of focus on Wednesday, December 15.
Geert Vanden Wijngaert/AP
A Russian tank seen during a military drill at Molkino training ground in the Krasnodar region on Tuesday, December 14.
A Russian tank seen during a military drill at Molkino training ground in the Krasnodar region on Tuesday, December 14.
AP
A Russian serviceman looks through binoculars as he takes part in drills at the Kadamovskiy firing range on Tuesday, December 14.
A Russian serviceman looks through binoculars as he takes part in drills at the Kadamovskiy firing range on Tuesday, December 14.
AP
A Ukrainian serviceman walks out of a dugout on the frontline with Russia-backed separatists near the village of Pesky, Donetsk region, on December 14.
A Ukrainian serviceman walks out of a dugout on the frontline with Russia-backed separatists near the village of Pesky, Donetsk region, on December 14.
Anatolii Stepanov/AFP/Getty Images
Ukrainian soldiers walk toward a destroyed building in Marinka on December 8.
Ukrainian soldiers walk toward a destroyed building in Marinka on December 8.
Brendan Hoffman/Getty Images
Ukrainian soldiers talk and drink tea in in Marinka on December 8.
Ukrainian soldiers talk and drink tea in in Marinka on December 8.
Brendan Hoffman/Getty Images
A destroyed police impound yard is seen in Marinka on December 8.
A destroyed police impound yard is seen in Marinka on December 8.
Brendan Hoffman/Getty Images
Ukrainian soldiers wait in a building near the front line in Marinka on December 8.
Ukrainian soldiers wait in a building near the front line in Marinka on December 8.
Brendan Hoffman/Getty Images
Students play while waiting for the bus after school in Krasnohorivka, Ukraine, on December 8.
Students play while waiting for the bus after school in Krasnohorivka, Ukraine, on December 8.
Brendan Hoffman/Getty Images
Russian President Vladimir Putin meets with US President Joe Biden via video conference to <a href="index.php?page=&url=https%3A%2F%2Fwww.cnn.com%2Fpolitics%2Flive-news%2Fbiden-putin-call-ukraine-russia%2Findex.html" target="_blank">discuss the escalating tensions</a> on Tuesday, December 7.
Russian President Vladimir Putin meets with US President Joe Biden via video conference to discuss the escalating tensions on Tuesday, December 7.
Mikhail Metzel/Pool/AP
A Ukrainian soldier rests near a fighting position on the line of separation from pro-Russian rebels near Katerinivka, Ukraine, on December 7.
A Ukrainian soldier rests near a fighting position on the line of separation from pro-Russian rebels near Katerinivka, Ukraine, on December 7.
Andriy Dubchak/AP
Ukrainian soldiers walk near Katerinivka on December 7.
Ukrainian soldiers walk near Katerinivka on December 7.
Andriy Dubchak/AP
In this handout photo from the Ukrainian Presidency, Ukrainian President Volodymyr Zelensky visits the front line in Donbass, Ukraine, on Monday, December 6.
In this handout photo from the Ukrainian Presidency, Ukrainian President Volodymyr Zelensky visits the front line in Donbass, Ukraine, on Monday, December 6.
Ukrainian Presidency/Anadolu Agency/Getty Images
A Ukrainian soldier cooks in a shelter near Debaltsevo, Ukraine, on Friday, December 3.
A Ukrainian soldier cooks in a shelter near Debaltsevo, Ukraine, on Friday, December 3.
Andriy Dubchak/AP
A Ukrainian soldier looks out from a fighting position on the line of separation from pro-Russian rebels near Debaltsevo on December 3.
A Ukrainian soldier looks out from a fighting position on the line of separation from pro-Russian rebels near Debaltsevo on December 3.
Andriy Dubchak/AP
Ukrainian soldiers walk in a trench along the front line in Avdiika, Ukraine, on Thursday, December 2.
Ukrainian soldiers walk in a trench along the front line in Avdiika, Ukraine, on Thursday, December 2.
Brendan Hoffman/The New York Times/Redux
A Ukrainian soldier checks the situation near Svetlodarsk, Ukraine, on December 2.
A Ukrainian soldier checks the situation near Svetlodarsk, Ukraine, on December 2.
Anatolii Stepanov/EPA-EFE/Shutterstock
A Ukrainian soldier enters a building in Avdiivka, Ukraine, on Wednesday, December 1.
A Ukrainian soldier enters a building in Avdiivka, Ukraine, on Wednesday, December 1.
Brendan Hoffman/The New York Times/Redux
A Ukrainian soldier's mud-covered boots are seen at a front-line position in the industrial zone of Avdiivka on December 1.
A Ukrainian soldier's mud-covered boots are seen at a front-line position in the industrial zone of Avdiivka on December 1.
Brendan Hoffman/The New York Times/Redux
Ukrainian soldiers are seen at a base built into the skeletal remains of a tire factory in Avdiivka on December 1.
Ukrainian soldiers are seen at a base built into the skeletal remains of a tire factory in Avdiivka on December 1.
Brendan Hoffman/The New York Times/Redux
Ukrainian soldiers relax in their barracks in Avdiivka on December 1.
Ukrainian soldiers relax in their barracks in Avdiivka on December 1.
Brendan Hoffman/The New York Times/Redux
This satellite image shows Russian troops in Yelna, Russia, on November 9. <a href="index.php?page=&url=https%3A%2F%2Fwww.cnn.com%2F2021%2F11%2F04%2Feurope%2Frussia-ukraine-military-buildup-intl-cmd%2Findex.html" target="_blank">Satellite photos</a> taken that month revealed Russian hardware -- including self-propelled guns, battle tanks and infantry fighting vehicles -- on the move at a training ground roughly 186 miles (300 km) from the border.
This satellite image shows Russian troops in Yelna, Russia, on November 9. Satellite photos taken that month revealed Russian hardware -- including self-propelled guns, battle tanks and infantry fighting vehicles -- on the move at a training ground roughly 186 miles (300 km) from the border.
Maxar Technologies/AP