Ukraine's President Volodymyr Zelenskiy said on Thursday that an escalation of tension in Ukraine's eastern regions showed Russia was trying to create a threatening atmosphere for ceasefire talks.
"Playing with muscles in the form of military exercises and possible provocations along the border is a traditional Russian affair," Mr Zelenskiy said in a Telegram message.
"In this way, it seeks to create an atmosphere of threat and, at the same time, of pressure during the ceasefire negotiations and for peace as our value."
Moscow 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 large Russian military build-up near eastern Ukraine.
Unverified social media footage has suggested Russia has been moving large numbers of tanks, armoured personnel carriers and other equipment to regions that border Ukraine and to Crimea, which Moscow annexed from Kiev in 2014.
The Russian activity poses an early challenge to US President Joe Biden's administration, which this week held calls with senior Ukrainian officials in a public show of support for President Volodymyr Zelenskiy's government.
On Thursday, State Department Spokesperson Ned Price said the US remains very concerned with recent developments.
“We can’t forget Russia’s ongoing aggression in Ukraine…we’re absolutely concerned by recent escalations of Russian aggressive and provocative actions in eastern Ukraine,” Mr Price said.
US Secretary of Defence Lloyd Austin called his Ukrainian counterpart Andrii Taran on Thursday.
“Secretary Austin reaffirmed unwavering US support for Ukraine’s sovereignty, territorial integrity, and Euro-Atlantic aspirations."
"He condemned recent escalations of Russian aggressive and provocative actions in eastern Ukraine and offered condolences to Minister Taran on the deaths of four Ukrainian soldiers on March 26,” the Pentagon statement said.
The US defence chief reiterated commitment to building the capacity of Ukraine’s forces to defend more effectively against Russian aggression.
This was the fourth senior US official to call the Ukraine in last 24 hours following calls from Secretary of State Tony Blinken, National Security Advisor Jake Sullivan and Chairman of the Joint Chief of Staff Mark Milley on Wednesday.
“Partly it is the usual tactics, turning up and down the conflict to create instability, to show that Russia is a key player," an EU diplomat said.
"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 Kiev 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 Kiev 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 its ambassadors met to discuss a recent increase in violence in the eastern Donbass region.
There Ukrainian troops have battled Russian-backed separatist forces in a conflict Kiev estimates has killed 14,000 people since 2014.
Ukrainian Foreign Minister Dmytro Kuleba accused Russia of "systemic aggravation" of the situation in Donbass and Crimea.
Kremlin spokesman Dmitry Peskov called for calm on the build-up.
"This should not bother anyone and it poses no threat to anyone," Mr Peskov said.
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," Mr Peskov said. "It all obliges us to be vigilant.".
Russian Foreign Minister Sergey 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," Mr Lavrov said.
"Russian President Putin said 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."
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," the official said.
"Allies are also concerned about Russian violations of the July 2020 ceasefire that led to the death of four Ukrainian soldiers last week."
Ukraine, western countries and Nato accuse Russia of sending troops and heavy weapons to prop up its proxies in Donbass, who seized a large part 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.
Gen Mark Milley, Chairman of the US Joint Chiefs of Staff, spoke to Ukrainian Commander-in-Chief Ruslan Khomchak to underscore that "Ukraine is a key partner to Nato", his office said.
Ukrainian military intelligence accused Russia of engineering provocations to create a pretext to send more Russian military to Donbass.
"In addition, an attempt to advance Russian occupation forces deep into Ukraine is not ruled out," it said.