File photo: A tank of pro-Russian militants rides during a military exercise not far from a city in Ukraine Photograph:( AFP )
Russia has reportedly amassed troops and moved tanks near the Ukrainian border in recent days, as a ceasefire in eastern Ukraine looks increasingly shaky.
The US State Department called on Russia to refrain from escalating tensions with Ukraine, in comments to reporters.
"We call on Russia to refrain from escalatory actions," State Department spokesman Ned Price said.
"We've asked Russia for an explanation of these provocations," Price said. "But more importantly, what we have signalled with our Ukrainian partners is a message of reassurance."
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Russia has reportedly amassed troops and moved tanks near the Ukrainian border in recent days, as a ceasefire in eastern Ukraine looks increasingly shaky. The movements are the latest point of tension between Russia and the US less than three months after US President Joe Biden took office.
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The comments by Ned Prince followed a telephone call on Friday in which Biden reassured his Ukrainian counterpart, Volodymyr Zelinskiy, of "unwavering support" in Ukraine's confrontation with Russia-backed separatists holding parts of the country's eastern Donbas region.
Russia on Monday denied that Russian military movements posed a threat to Ukraine and dismissed fears of a buildup even as it warned that it would respond to new Ukrainian sanctions against Russian companies.
White House press secretary Jen Psaki told reporters that "recent escalations of Russian aggression and escalation in eastern Ukraine" is "something we're watching closely."
(With inputs from agencies)