President Joe Biden has warned Ukraine's leader that there is a "distinct possibility" Russia could take military action against the country next month.
Likewise, the Kremlin has voiced a similarly grim note, saying it saw "little ground for optimism" in resolving the crisis after the US this week again rejected Russia's main demands.
Russian officials said dialogue was still possible to end the crisis, but Mr Biden on Thursday again offered a stark warning amid growing concerns that Russian President Vladimir Putin will give the go-ahead for a further invasion of Ukrainian territory in the not-so-distant future.
The White House said on Thursday, Mr Biden's comments to Ukraine's President Volodymyr Zelenskyy in a phone call amplified concerns that administration officials have been making for some time.
White House National Security Council spokesperson Emily Horne said: "President Biden said that there is a distinct possibility that the Russians could invade Ukraine in February.
"He has said this publicly and we have been warning about this for months."
Tensions have soared in recent weeks, as the US and its NATO allies expressed concern that a build-up of about 100,000 Russian troops near Ukraine signalled that Moscow planned to invade its ex-Soviet neighbour.
Russia has denied any such designs and has presented a series of demands it says will improve security in Europe.
Russia has manoeuvred about 100,000 troops to near the border with Ukraine, although Moscow has denied it plans to invade its former Soviet neighbour.
A UK minister, however, said it is "not too late" for President Putin to "de-escalate" tensions on the Ukraine border, amid international fears of an invasion.
Chris Philp, a minister in the Department for Digital, Culture, Media and Sport, told Sky News the deep concern at the build-up of Russian troops around the Ukrainian border had reached "unprecedented levels".
Mr Philp added: "I think President Biden and our Prime Minister are right to warn President Putin that the consequences, if they do invade Ukraine, will be very, very serious indeed for Russia in terms of sanctions."
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"In the United Kingdom, we are doing everything we can to support the Ukrainians, including supplying military equipment that would help them – and training that goes with that – to prevent tanks, Russian tanks, from entering Ukrainian territory.
"We are urging Russia to get to the table, to discuss issues that are relevant and to de-escalate the situation."
When speaking to his Ukrainian counterpart, Mr Zelenskyy, Mr Biden reiterated American and allied support, including recent deliveries of US military aid.
President Biden warned Mr Zelenskyy that the US believed there was a high degree of likelihood Russia could invade when the ground freezes and Russian forces could attack Ukrainian territory from north of Kyiv, according to two people familiar with the conversation who were not authorised to comment publicly.
Mr Zelenskyy tweeted that he and President Biden also discussed the possibility of additional financial support for Ukraine.
The White House said President Biden told Mr Zelenskyy he was "exploring additional macroeconomic support to help Ukraine's economy" as it comes under pressure.
The US has also reassured the EU it will help them find alternative gas supplies if Russia cuts them off, with the EU nations relying on Russia for around a third of their gas supplies.
This comes after the US threatened to block the completion of the Nord Stream 2 pipeline from Russia to Germany if Moscow makes a move on Ukraine.
US State Department spokesperson Ned Price reiterated this point, saying: "I want to be very clear: if Russia invades Ukraine one way or another, Nord Stream 2 will not move forward."