Russia has the troops in place to invade Ukraine "at any time" and American citizens should leave within the next 48 hours, the US has warned.
Volunteers of the 112th Territorial Defence Brigade of Kiev in a military training on the outskirts of the city. Photo: AFP
An invasion could start with aerial bombing that would make departures difficult and endanger civilians, the White House said on Friday.
A host of other countries have also urged their nationals to leave Ukraine.
Moscow has repeatedly denied any plans to invade Ukraine despite massing more than 100,000 troops near the border.
Russia's foreign ministry on Friday accused Western countries of spreading false information.
US National Security Adviser Jake Sullivan said Russian forces were now "in a position to be able to mount a major military action" in remarks seen as a clear escalation in the urgency of warnings from US officials.
"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 [leaving] is prudent," he said.
Sullivan added that the administration did not know if Russian President Vladimir Putin had made a final decision to invade, but said that the Kremlin was looking for a pretext to justify military action, which he said could start with intense aerial bombardment.
His comments came as US officials warned of a further build-up of Russian troops at Ukraine's borders over the past week and planned Russian military exercises in the Black Sea in the coming days.
Earlier, US Secretary of State Antony Blinken said the increase in Russian forces at the border was "very troubling signs of Russian escalation".
"We're in a window when an invasion could begin at any time, and to be clear, that includes during the Olympics [which end on 20 February]," Blinken said.
President Biden has said that he would not send troops to rescue any citizens left stranded in the event of Russian action.
On Friday, the US president hosted a video call with transatlantic leaders in which they agreed on co-ordinated action to inflict severe economic consequences on Russia if it invaded Ukraine.
The US also said it was deploying a further 3,000 troops from Fort Bragg, North Carolina, to Poland, and that they were expected to arrive there next week. The troops will not fight in Ukraine, but will ensure the defence of US allies.
Moscow has begun massive military drills with neighbouring Belarus, and Ukraine has accused Russia of blocking its access to the sea.
The Kremlin says it wants to enforce "red lines" to make sure that its former Soviet neighbour does not join Nato.
Nato Secretary-General Jens Stoltenberg has said the bloc was "united and prepared for any scenario".
John Herbst, US ambassador to Ukraine between 2003 and 2006, said that despite the US government's warnings, he believes a full-scale Russian invasion of Ukraine remains unlikely.
Among other countries calling on citizens to leave are the UK, the Netherlands, Latvia, Japan and South Korea.
The British foreign office said all UK nationals "should leave now while commercial means are still available".
In its warning, Latvia cited "a serious threat to security posed by Russia".
The current tensions come eight years after Russia annexed Ukraine's southern Crimea peninsula. Since then, Ukraine's military has been locked in a war with Russian-backed rebels in eastern areas near Russia's borders.
Russian naval drills took place in Crimea on Friday, while 10 days of military exercises continued in Belarus, to the north of Ukraine.
There are fears that if Russia tries to invade Ukraine, the exercises put the Russian military close to the Ukrainian capital, Kiev, making an attack on the city easier. Russia says its troops will return to their permanent bases after the drills end.
Moscow says it cannot accept that Ukraine - a former Soviet republic with deep social and cultural ties with Russia - could one day join the Western defence alliance Nato and has demanded that this be ruled out.
Russia has been backing a bloody armed rebellion in Ukraine's eastern Donbas region since 2014. Some 14,000 people - including many civilians - have died in fighting since then.
There is some suggestion that a renewed focus on the so-called Minsk agreements - which sought to end the conflict in eastern Ukraine - could be used as a basis to defuse the current crisis.
Ukraine, Russia, France and Germany backed the accords in 2014-2015.
-BBC