President Joe Biden said that the US and its allies would "respond decisively" if Russia further invaded Ukraine, in a phone call with Ukrainian President Volodymyr Zelensky.
"President Biden made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine," said a White House statement about the call on Sunday.
Mr Biden also reaffirmed Washington's commitment to Ukraine's "sovereignty and territorial integrity" and to the principle of "nothing about you without you," the statement said.
The US president also expressed support for measures to de-escalate tensions in the Ukrainian separatist region of Donbas and "active diplomacy to advance the implementation of the Minsk Agreements" in support of the so-called Normandy format, a series of Franco-German mediated talks aimed at achieving peace in the eastern Ukraine region.
On Thursday, Mr Biden spoke with Russian President Vladimir Putin over the phone for almost an hour, addressing the Ukraine conflict among other key topics.
The call came ahead of talks in Geneva on 9 and 10 January, during which diplomats from both countries will meet to discuss the stand-off. Russia and NATO are planning hold separate talks two days later, on 12 January.
For weeks, the West has been concerned about reports that Russia has massed tens of thousands of soldiers in areas not far from Ukraine, triggering fears of a Russian invasion.
A Ukrainian soldier walks at the line of separation from pro-Russian rebels near Popasna in the Donetsk region, Ukraine. Source: AAP
Russia rejects the charge, and in turn accuses Ukraine of having moved more soldiers to the eastern areas held by pro-Russian separatist.
The developments recall the events of 2014, when Russia annexed Ukraine's Crimean peninsula and began its ongoing support for separatists in eastern Ukraine.
