The White House said that a Russian invasion of Ukraine could come within the week, possibly within the next two days, and urged Americans to leave the country now.

US national security adviser Jake Sullivan said the United States did not have definitive information that an invasion has been ordered by Russian President Vladimir Putin.

But he said all the pieces were in place for a major military operation that could start “rapidly”.

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

”The strong possibility of action, the distinct possibility of action, in a relatively near term time frame… is backed up by our view of what’s happening on the ground.”

“We continue to see signs of Russian escalation, including new forces arriving at the Ukrainian border,” he added, ramping up the urgency of earlier US warnings. “As we’ve said before, we are in the window when an invasion could begin at any time should Vladimir Putin decide to order it.

“The risk is high enough and the threat is now immediate enough that prudence demands that it is the time to leave now.”

“We are not saying that a decision has been taken by President Putin,” Mr Sullivan added.

“What we are saying is that we have a sufficient level of concern based on what we are seeing on the ground, and what our intelligence analysts have picked up, that we are sending this clear message.”

Russia is holding massive war games in neighbouring Belarus and insisting that the highly strained relations is not its fault.

Russia says it has no plans to invade but wants the West to keep Ukraine and other former Soviet countries out of Nato.

Meanwhile, the Pentagon is sending another 3,000 combat troops to Poland to join 1,700 who already are assembling there in a demonstration of American commitment to Nato allies.

A defence official said the additional soldiers will depart their post at Fort Bragg, North Carolina, over the next couple of days and should be in Poland by early next week.

They are the remaining elements of an infantry brigade of the 82nd Airborne Division.

In addition to the US troops deploying to Poland, about 1,000 US soldiers based in Germany are shifting to Romania in a similar mission of reassurance to a Nato ally.