NATO commander: Russian troops, tanks, artillery move into Ukraine

"We've stressed many a time there are no real facts behind the acts of shaking the air by Brussels officials," Konashenkov said, referring to the Belgian capital, where NATO is based.

A day earlier, Breedlove said Russia has moved "forces that are capable of being nuclear" into Crimea, which was Ukrainian territory until being folded into Russia on the heels of a government turnover earlier this year. Crimea borders southeastern Ukraine, where much of the current unrest is focused and into which Russian troops allegedly have moved.

"Whether they are [nuclear-equipped] or not, we don't know," Breedlove, the top U.S. general in Europe, said Tuesday. "But they do have the kind of equipment there that could support that mission if required."

Russian President Vladimir Putin has alluded to his country's nuclear arsenal, amid criticism of Russia's actions. In late August, state-run TASS reported that he told a youth forum, "I want to remind you that Russia is one of the most powerful nuclear nations. This is a reality, not just words."

Asked specifically about possible nuclear movement into Crimea, Putin's spokesman Dmitry Peskov said, "NATO releases such statements almost on a daily basis. We have no intention to react and comment on them."

Russian Defense Minister Sergey Shoigu did say Wednesday that his military is considering forming "a full-fledged and self-sufficient" unit in Crimea. Those troops' purpose would be "to ensure the military security of the country and its allies," said Shoigu, according to another TASS report.

"In many respects, this is connected with the situation in Ukraine, with fomentation of anti-Russian moods on the part of NATO and reinforcement of foreign military presence in the immediate vicinity to our borders," the defense minister said.

The United Nations Security Council held an emergency meeting Wednesday afternoon on the worsening situation, where one official warned of a return to serious fighting.

"We are deeply concerned over the possibility of a return to full-scale fighting," Jens Anders Toyberg-Frandzen, U.N. assistant secretary-general for political affairs, told the Security Council.

The alternative to all-out war, still a "catastrophe for Ukraine," would be months of continuing "low-level battles marked by periods of increased hostilities and further casualties," Toyberg-Frandzen said.

All these accusations and actions come amid intensified fighting of late in southeastern Ukraine.

Pro-Russian rebels claim control of parts of that region, despite a push by Ukrainian forces to defeat them. The two sides signed a ceasefire deal in September, raising hope that the months-long conflict was nearing an end.

But now, fighting has returned to levels that preceded the ceasefire, a British security source who has detailed knowledge of the matter told CNN.

As Breedlove said Tuesday, "The ceasefire is in name only at this point."

Ukrainian Defense Minister Stepan Poltorak said his government needs to be ready for combat operations, especially amid concerns that separatists and their Russian allies are building up their forces.

Ukrainian defense spokesman Andriy Lysenko said late last week that 32 tanks, 16 D-30 howitzers and 30 KamAZ heavy trucks crossed from Russia past a border checkpoint and headed toward the volatile Luhansk region. Another Ukrainian official, Dmytro Tymchuk, alleged the "armored column" consisted of "a battalion tactical group of the Russian Armed Forces," according to Ukrinform state news agency.

Talking at a government meeting Wednesday, Poltorak said that "of course, we expect .... actions" by those forces moving into and around southeastern Ukraine.

He added, "I think the main task is to prepare for fighting."