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.

An invasion could start with aerial bombing that would make departures difficult and endanger civilians, the White House said on Friday.

Moscow has repeatedly denied any plans to invade Ukraine despite massing more than 100,000 troops near the border.

A host of other countries have also urged their nationals to leave Ukraine.

They include the UK, Canada, the Netherlands, Latvia, Japan and South Korea.

Russia's foreign ministry has 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.

Earlier, US Secretary of State Antony Blinken said the increase in Russian forces at the border was "very troubling signs of Russian escalation".