ITV Correspondent Dan Rivers spoke to an American expat in Kyiv, who explained why he doesn't want to leave Ukraine despite the threat of Russia
US National Security Adviser Jake Sullivan urged Americans in the former Soviet state to leave within the next 48 hours, warning it may be their last chance to get out if an invasion happens - something he said was an "immediate " threat.
He warned in a press conference that a possible Russian invasion could start before the end of the Winter Olympics, which concludes on February 20.
Mr Sullivan said he "could not pinpoint" a day but it is a "very, very distinct possibility" action from Moscow is imminent due to what they believe is "happening on the ground".
Russia has already amassed around 10,000 troops on the border with Ukraine. New intelligence showed there had been another increase in troops as well as the beginning of a major Russian military exercise in neighbouring Belarus.
Where Russian troops are massed across the Ukraine border
The UK also earlier told its citizens to leave Ukraine immediately "while commercial means are still available".
Speaking from the White House on Friday, Mr Sullivan said: “We continue to see signs of Russian escalation including new forces arriving at the Ukrainian border.
“As we’ve said before, we are in the window of when an invasion begins at any time should Vladimir Putin decide to order it.
“I will not comment on the details of our intelligence information but I do want to be clear it could begin during the Olympics despite a lot of speculation that it will only happen after the Olympics.
“Any American in Ukraine should leave as soon as possible and in any event in the next 24 to 48 hours. We obviously cannot predict the future, we don’t know exactly what is going to happen, but the risk is now high enough and the threat is now immediate enough that this is prudent.”
US National Security Adviser Jake Sullivan: "If a Russia attack on Ukraine precedes it is likely to begin with aerial bombing and missile attacks, that could obviously kill civilians without regard to their nationality"
The White House said there is no conclusive intelligence to confirm President Putin had made a decision to invade - but he has assembled all the elements to do so quickly.
“We believe he very well may give the final ‘go order’,” Mr Sullivan said. “That is a very distinct possibility. It may well happen soon.”
It is likely a Russian attack on Ukraine would begin with aerial bombing, missile attacks and then a ground invasion of “a massive force” that could "kill civilians without regard to their nationality", added Mr Sullivan.
Warning US citizens to leave the country immediately, he said: “If you stay, you are assuming risk with no guarantee that there will be any other opportunity to leave and no prospect of a US military evacuation in the event of a Russian evasion".
“With virtually no notice, communications to arrange a departure could be severed and commercial transit halted,” he warned.
'Emotions running desperately high now on both sides' - Emma Murphy in the US, Rachel Younger in Moscow and Global Security Editor Rohit Kachroo gives some insight into the frenzy over the Ukraine-Russia situation
Russian Foreign Ministry spokeswoman Maria Zakharova retorted immediately: “The hysteria of the White House is more indicative than ever.
"The Anglo-Saxons need a war. At any cost. Provocations, misinformation and threats are a favorite method of solving their own problems.”
President Biden has ordered a further 3,000 US soldiers to Poland, on top of 1,700 who are already on their way there.
Mr Biden said US troops will not enter Ukraine to contest any Russian invasion, but he has bolstered his military presence in Europe as reassurance to allies on NATO’s eastern flank.
Ms Zakharova responded by saying: “NATO keeps building up its presence near Russia’s borders and exacerbates the situation around Ukraine to create a pretext for that.”
The stark US and UK warnings came as ongoing emergency diplomatic efforts failed to calm tensions.
The most recent talks in Moscow collapsed on Thursday between UK Foreign Secretary Liz Truss and her Russian counterpart Sergei Lavrov.
There had been mounting pressure on Ms Truss to deliver a breakthrough but Mr Lavrov said speaking to her was like "the deaf talking to the blind", before he walked out of a press conference leaving her behind at the podium.
Mr Biden held a call with other world leaders to discuss the escalating crisis on Friday, with Boris Johnson telling leaders he fears for the security of Europe, according to Downing Street.
After the virtual call, a No 10 spokesman said: “The Prime Minister told the group that he feared for the security of Europe in the current circumstances.
“He impressed the need for Nato allies to make it absolutely clear that there will be a heavy package of economic sanctions ready to go, should Russia make the devastating and destructive decision to invade Ukraine.
“The Prime Minister added that President Putin had to understand that there would be severe penalties that would be extremely damaging to Russia’s economy, and that Allies needed to continue with efforts to reinforce and support the Eastern frontiers of Nato.
“He urged the leaders to work together to deliver economic and defensive support to Ukraine.
“The leaders agreed that if President Putin deescalated, there was another way forward, and they pledged to redouble diplomatic efforts in the coming days.”