Several US allies including Canada, Australia, New Zealand, Norway and Denmark also asked their citizens to leave Ukraine.

02:38

‘We’re scared’: fear and uncertainty on Ukraine’s front lines

‘We’re scared’: fear and uncertainty on Ukraine’s front lines

Biden has said the US military will not enter a war in Ukraine, but he has promised severe economic sanctions against Moscow, in concert with international allies.

Timing of possible Russian military action remains a key question.

The US picked up intelligence that Russia is looking at Wednesday as a target date, according to a US official familiar with the findings.

Australia, New Zealand ask citizens in Ukraine to leave immediately

The official, who was not authorised to speak publicly and did so only on condition of anonymity, would not say how definitive the intelligence was, and the White House publicly underscored that the US does not know with certainty whether Putin is committed to invasion.

However, US officials said anew that Russia’s build-up of offensive air, land and sea firepower near Ukraine has reached the point where it could invade on short notice.

Biden’s national security adviser, Jake Sullivan, urged all Americans in Ukraine to leave within the next 48 hours, emphasising that they should not expect the US military to rescue them in the event that air and rail transport is severed after a Russian invasion.

Biden urges Americans to leave Ukraine ‘now’ as Russian invasion fears mount

Sullivan said Russian military action could start with missile and air attacks, followed by a ground offensive.

“Yes, it is an urgent message because we are in an urgent situation,” he told reporters at the White House.

“Russia has all the forces it needs to conduct a major military action,” Sullivan said, adding, “Russia could choose, in very short order, to commence a major military action against Ukraine.”

He said the scale of such an invasion could range from a limited incursion to a strike on Kyiv, the capital.

03:18

Ukrainians train in self-defence in face of Russian threat as Western leaders try to defuse crisis

Ukrainians train in self-defence in face of Russian threat as Western leaders try to defuse crisis

In addition to the more than 100,000 ground troops that US officials say Russia has assembled along Ukraine’s eastern and southern borders, the Russians have deployed missile, air, naval and special operations forces, as well as supplies to sustain a war.

Earlier this week Russia moved six amphibious assault ships into the Black Sea. Then on Saturday, more than 30 ships from the Russian Black Sea fleet started training exercises near the Crimea peninsula as part of wider navy drills, RIA news agency reported.

RIA said that more than 30 Russian ships have left the ports of Sevastopol and Novorossiisk “in accordance to the plans of the drills” near Crimea, which Russia annexed from Ukraine in 2014. It said the aim of the drills was the defence of the coast of Crimea, the outposts of the Black Sea fleet as well as the economy sector and naval communications.

Russia announced last month its navy would stage a sweeping set of exercises involving all its fleets this month and next, from the Pacific to the Atlantic, the latest show of strength in a surge of military activity during the stand-off with the West.

Ukraine crisis: Russia sends 6 landing warships to Black Sea

Sullivan’s stark warning accelerated the projected time frame for a potential invasion, which many analysts have believed was unlikely until after the Winter Olympics in China end on February 20.

Sullivan said the combination of a further Russian troop build-up on Ukraine’s borders and unspecified intelligence indicators have prompted the administration to warn that war could begin any time.

“We can’t pinpoint the day at this point, and we can’t pinpoint the hour, but that is a very, very distinct possibility,” Sullivan said.

Pentagon press secretary John Kirby said Defence Secretary Lloyd Austin conferred by phone with several of his Nato counterparts. Echoing Sullivan’s public remarks, Austin told them a Russian invasion of Ukraine “could begin at any time,” Kirby said.

Biden has said US troops will not enter Ukraine to contest any Russian invasion, but he has bolstered the US military presence in Europe as reassurance to allies on Nato’s eastern flank.

On Friday the Pentagon said Biden ordered a further 3,000 soldiers to Poland, on top of 1,700 who are on their way there. Together they form an infantry brigade of the 82nd Airborne Division. The US Army also is shifting 1,000 soldiers from Germany to Romania, which like Poland shares a border with Ukraine.

General Mark Milley, chairman of the Joint Chiefs of Staff, spoke by phone on Friday with his Russian counterpart, General Valery Gerasimov. Milley’s office provided no details beyond saying the two men discussed “several security-related issues of concern”.

‘Confrontation not inevitable’: US tells Quad allies to stand up to China

Milley also had phone calls with several of his counterparts from Nato countries, including Canada, France, Germany, Britain, Italy, Poland and Romania.

Secretary of State Antony Blinken, who was travelling in Australia, was the first senior US official to say publicly that an invasion could come before the end of the Olympics.

Sullivan would not discuss the intelligence details behind the US assessment and denied a report that American officials believe Putin has made the decision to invade. But he said US officials believe there is “a strong possibility” of an invasion.

“We believe he very well may give the final go order,” Sullivan said. “It may well happen soon.”