MOSCOW/KYIV - Russia warned on Thursday that a serious escalation in the conflict in Donbass could "destroy" Ukraine as NATO voiced concern over what it said was a big Russian military build-up near eastern Ukraine.Unverified social media footage has suggested Russia has been moving large quantities of tanks, armoured personnel carriers and other equipment to regions that border Ukraine as well as to Crimea, which Moscow annexed from Ukraine in 2014.The Russian activity poses an early challenge to U.S. President Joe Biden's administration, which this week held phone calls with senior Ukrainian officials in a public show of support for President Volodymyr Zelenskiy's government. "Partly, it is the usual tactics, turning up and down the conflict to create instability, to show that 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 peace deal over Donbass. Relations are further strained by the fate of the Nord Stream 2 gas pipeline to Germany, which Kyiv and Washington want to halt, and by a crackdown in Ukraine on a prominent businessman close to Russia.NATO said it was concerned about the Russian military build-up as NATO ambassadors met to discuss a recent spike in violence in the eastern Donbass region, where Ukrainian troops have battled Russian-backed separatist forces in a conflict Kyiv estimates has killed 14,000 people since 2014.U.S. Defense Secretary Lloyd Austin spoke with his 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 that 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."NATO CONCERN A NATO official told Reuters that Moscow was undermining peace efforts in eastern Ukraine."Allies shared their concerns about Russia's recent large scale military activities in and around Ukraine. Allies are also concerned about Russian violations of the July 2020 ceasefire that led to the death of four Ukrainian soldiers last week," the official said.Ukraine, Western countries and NATO accuse Russia of sending troops and heavy weapons to prop up its proxies in Donbass who seized a swathe of eastern Ukraine in 2014.Russia says it only provides political and humanitarian support to separatist fighters in what it casts as an internal conflict.Ukrainian military intelligence accused Russia of engineering provocations to create a pretext to send additional Russian military units to Donbass."In addition, an attempt to advance Russian occupation forces deep into Ukraine is not ruled out," it said.