Russia says a serious escalation in the conflict in Donbass could "destroy" Ukraine as NATO voices concern over what it describes as a big Russian military build-up to the east.
Unverified social media footage suggests Russia has been moving large quantities of tanks, armoured personnel carriers and other equipment to regions that border Ukraine as well as Crimea, which Moscow annexed from Ukraine in 2014.
The activity poses an early challenge to US President Joe Biden's administration, which this week held phone calls with senior Ukrainian officials in a show of support for President Volodymyr Zelenskiy.
"Partly, it is the usual tactics, turning up and down the conflict to create instability, to show Russia is a key player," said one EU diplomat.
"We cannot exclude that Biden's presidency is part of the Russian calculus, that it's time for Moscow to show a bit of muscle."
Moscow and Kyiv have long accused each other of failing to implement a Donbass peace deal.
Relations are further strained by the fate of the Nord Stream 2 gas pipeline to Germany, which Kyiv and Washington want to halt, and a crackdown in Ukraine on a prominent businessman close to Russia.
NATO said on Thursday it was concerned about the Russian military build-up as its ambassadors met to discuss a recent spike in violence in the eastern Donbass, where Ukrainian troops have battled Russian-backed separatist forces in a conflict Kyiv estimates has killed 14,000 people since 2014.
US Defense Secretary Lloyd Austin spoke with Ukrainian counterpart Andrii Taran and "condemned recent escalations of Russian aggressive and provocative actions in eastern Ukraine", the Pentagon said.
"I reaffirmed our unwavering support for Ukraine's sovereignty, territorial integrity, and Euro-Atlantic aspirations," Austin said on Twitter.
Ukrainian Foreign Minister Dmytro Kuleba accused Russia of a "systemic aggravation" of the security situation in Donbass and Crimea.
Kremlin spokesman Dmitry Peskov played down the build-up, saying "this should not bother anyone and it poses no threat to anyone".
Russia was taking measures to ensure the security of its own borders, he said.
"There is increased activity on the perimeter of Russia's borders by NATO, other alliances, individual countries - it all obliges us to be vigilant," said Peskov.
Russian Foreign Minister Sergei Lavrov said most of Ukraine's military appeared to understand the danger of a "hot conflict" in Donbass.
"I very much hope that they will not be 'incited' by politicians, who in turn will be 'incited' by the West, led by the United States," said Lavrov.
"Russian President Putin said (this) not long ago but this statement is still relevant today, that those who would try to start a new war in Donbass - will destroy Ukraine."