Russia could invade Ukraine on Wednesday after US spies reportedly discovered details about an invasion plan.
It comes as Vladimir Putin pulls his diplomats out of the Ukrainian capital Kyiv as the threat of war between the two countries rises.
British, American and German citizens have been told to leave Ukraine immediately as more than 100,000 Russian troops mass on the border.
Fears are growing that flights out of the country may become difficult if the Russians start a deadly 'aerial bombardment of Kyiv'.
The US has also ordered non-emergency embassy staff to leave Ukraine, following previous withdrawals of diplomats by America and Britain.
A German newspaper claims US spies have received details about Russia's invasion plan, which reportedly sets the date of invasion as February 16.
The plans were passed to President Joe Biden's Government and have been discussed by NATO allies, according to Der Spiegel.
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Mr Biden's national security adviser Jake Sullivan said an attack before the end of the Winter Olympics on February 20 was a "credible prospect", assigning it a "very, very distinct possibility".
He warned a Russian attack on Ukraine could begin any day and would likely start with an air assault.
Mr Sullivan said new Russian forces were arriving at the border and are in a position to "mount a major military operation in Ukraine any day now", which could include a "rapid assault on the city of Kyiv" or on other parts of the country.
Speaking from the White House, he said Russia could choose "in very short order to commence a major military action against Ukraine", but stressed the US does not know whether Mr Putin has made a final decision.
Mr Sullivan said the "threat is now immediate enough" to urge Americans to leave Ukraine "as soon as possible and in any event in the next 24 to 48 hours".
It comes as the US State Department ordered non-emergency embassy staff to leave Ukraine on Saturday.
The State Department tweeted: "Despite the reduction in diplomatic staff, the core embassy team, our dedicated Ukrainian colleagues, and @StateDept and US personnel around the world will continue relentless diplomatic and assistance efforts in support of Ukraine's security, democracy, and prosperity."
Meanwhile, UK armed forces minister James Heappey said British troops helping with training in Ukraine will be leaving the country this weekend - and there will be no UK soldiers in Ukraine if Russia does invade.
Having sent UK personnel to train Ukrainians on the anti-tank missiles supplied by Britain, Mr Heappey said: "All of them will be withdrawn. There will be no British troops in Ukraine if there is to be a conflict there."
He added to BBC Radio 4's Today programme: "They will be leaving over the course of the weekend."
The minister previously warned Brits in Ukraine not to expect a military evacuation if Russia invades.
"British nationals should leave Ukraine immediately by any means possible and they should not expect, as they saw in the summer with Afghanistan, that there would be any possibility of a military evacuation," he said.
Speaking to BBC Breakfast, Mr Heappey said: "We are now confident that the artillery systems, the missile systems and the combat air are all in place that would allow Russia to launch - at no notice - an attack on Ukraine.
"And on that basis I think it is our responsibility to share with UK citizens our view that they should leave the country immediately while commercial means are still available.
"There will be a big difference between what they may have seen on their TV screens in Afghanistan over the summer and what may happen over the next week or so, and that is that the Royal Air Force will not be in a position to go in and to fly people out so they need to leave now by commercial means or drive out of Ukraine into a neighbouring country."
Mr Heappey hoped assurances from Moscow that Russia is not planning to invade remain true but noted the country could now launch an attack "very, very quickly".
Ireland's Department of Foreign Affairs has said it is monitoring the situation in Ukraine after the US and UK warned their citizens to leave the country.
It comes as Russia stages huge Black Sea war games in a powerful show of strength close to the Ukrainian coast.
Russian diplomats and consular staff today started leaving Ukraine, said a RIA Novosti source, in a sign that a full-blown conflict may be imminent.
The Russian embassy in the US said that a Putin order to invade was “not backed by the evidence”.
But Putin’s spokesman Dmitry Peskov said: "The cumulative effect of [NATO’s] expansion [in eastern Europe] has grown and festered, forcing Russia — to put it in the words of our president — to the edge of its red lines as far as national and security interests are concerned.”
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Putin and Biden are due to talk today, at the US president’s request. The Kremlin leader will also speak to French president Emmanuel Macron.
"The hysteria of the White House is more revealing than ever. The Anglo-Saxons need a war. At any price," said Russian Foreign Ministry spokeswoman Maria Zakharova.
"Our troops are still on our territory and I wonder if the US will invade Ukraine itself — someone has to, after such a panic campaign.”
The pro-Kremlin rebel leader of Donetsk People’s Republic, Denis Pushilin, indicated that he is near the point of formally seeking Russian intervention on Ukrainian territory.
He also claimed that he wants to see the whole of Ukraine as part of a new “union state” including Russia and Belarus - “that is how it is meant to be”.
A call for intervention in Donetsk and neighbouring Luhansk could give Putin the pretext for war that the West has warned he is seeking.
Pushilin accused Ukraine of deploying “heavy weapons” prohibited by the Minsk agreements to the line of contact in the Donbas.
He also claimed Polish and American mercenaries are arriving close to the unofficial border.
“The figure is becoming dangerous, it's not dozens, but already hundreds of people,” he alleged.
"In this situation, we are forced to be ready at any moment for the fact that Ukraine will proceed to offensive actions," he told Rossiya 24 television.
“We do not rule out that the situation may go along such a trajectory when we are forced to turn to the Russian Federation for help.”
He said: “The number of weapons that have been supplied from Western countries is hundreds of tonnes.
“Moreover, mercenaries and instructors, who at some point in time can simply retrain. This creates additional threats.
“Their unmanned aerial vehicles cause serious concern for us….the situation is difficult.”
Meanwhile, the Russian rouble fell the most in nearly two years on Friday after the US said Russia has amassed enough troops near Ukraine to launch a major invasion.
