S President Joe Biden warned his Russian counterpart Vladimir Putin that invading Ukraine would cause “widespread human suffering”, as Britons urged to flee the region began arriving back in the UK.
The White House account of the crisis call on Saturday said Mr Biden warned an attack would “diminish Russia’s standing” as the West pinned hopes on diplomacy to avert war.
They were said to have spoken for around an hour after French President Emmanuel Macron also shared a call with Mr Putin, with fears of an imminent attack heightening.
UK nationals in Ukraine are being urged by the Foreign Office to “leave now while commercial means are still available”.
Armed Forces minister James Heappey warned Russia is in a position to be able to attack “very, very quickly”, with an estimated 130,000 troops on Ukraine’s border.
But unlike when the Taliban seized Kabul, Mr Heappey stressed that the RAF would not be carrying out evacuations in the event of war in Ukraine.
The White House said: “President Biden was clear that, if Russia undertakes a further invasion of Ukraine, the United States together with our allies and partners will respond decisively and impose swift and severe costs on Russia.
“President Biden reiterated that a further Russian invasion of Ukraine would produce widespread human suffering and diminish Russia’s standing.”
Mr Biden told the Russian leader that the US and allies are prepared to engage in diplomacy – but are “equally prepared for other scenarios”, the White House said.
It was understood that Boris Johnson, who spoke to the Russian president earlier this month, does not have any calls with Moscow scheduled.
Defence Secretary Ben Wallace said it is “highly likely” that Russia will invade as he seemingly likened the last-minute flurry of diplomatic efforts to appeasement.
“It may be that he (Putin) just switches off his tanks and we all go home, but there is a whiff of Munich in the air from some in the West,” he said in an interview with the Sunday Times.
Foreign Secretary Liz Truss said she discussed her “acute concerns” that Russia “may launch further military aggression against Ukraine in coming days” during a call on Saturday with US secretary of state Antony Blinken.
“We agree Russia will face massive consequences for any invasion, including severe sanctions,” she said.
After UK nationals in Ukraine, thought to number in the low thousands, were ordered to leave on Friday night, passengers arrived on a flight to Gatwick Airport from Kyiv on Saturday afternoon.
Among them was 21-year-old Haider Ali from Birmingham, who said the warning had “caused quite a panic” with his fellow students at the Dnipro Medical Institute.
The Foreign Office’s order to leave was issued as intelligence and advice from experts on the ground suggested an increased threat level, with an invasion at some point deemed highly likely, the PA news agency understood.
Mr Heappey told BBC Breakfast: “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.”
British ambassador to Ukraine Melinda Simmons was remaining with a “core team” in Kyiv, but some embassy staff and their families were being withdrawn.
The Foreign Office’s order to flee was issued as intelligence and advice from experts on the ground suggested an increased threat level, with an invasion at some point deemed highly likely, PA understands.
Mr Wallace has said an invasion could come “at any time”, while US national security adviser Jake Sullivan said an attack before the end of the Winter Olympics on February 20 is a “credible prospect”.
Western leaders have threatened Moscow with a damaging package of sanctions in the event of a further incursion into Ukrainian soil.
Ukraine is not a Nato member and allies in the defence alliance have said they would not join fighting in Ukraine, but have bolstered forces in neighbouring nations and are threatening widespread sanctions.
Though the Kremlin insists it is not planning an invasion, US intelligence suggests Russia could fabricate a “false flag” pretext to attack.
Mr Heappey told BBC Radio 4’s Today programme that the UK personnel sent to train Ukrainians to use British-supplied anti-tank missiles will be “leaving over the course of the weekend”.