US National Security Advisor Jake Sullivan said on Friday that a Russian invasion of Ukraine "could begin at any time" including during the Beijing Winter Olympics.

"We continue to see signs of Russian escalation including new forces arriving at the Ukrainian border," Sullivan told reporters at the White House.

"An invasion could begin at any time should Vladimir Putin decide to order it," he said. "It could begin during the Olympics despite a lot of speculation that it will only happen after the Olympics."

The United States believes there is a "very distinct possibility" Russia will invade Ukraine but does not know if Vladimir Putin has made a "final decision," Sullivan added.

"The way that he has built up his forces and put them in place... makes it clear to us that there's a very distinct possibility that Russia will choose to act militarily," he stressed.

Sullivan went on to say that "our view is that we do not believe he has made any kind of final decision, or we don't know that he has made any final decision."

The US National Security Advisor predicted that a potentially imminent Russian attack on Ukraine could begin with aerial bombing, urging Americans to leave the country in the next 24-48 hours.

He also expected that US President Joe Biden would call his Russian counterpart Vladimir Putin.

"I would expect that President Biden will engage by telephone with President Putin, but I don't have anything to announce for you on that right now," Sullivan concluded.

Russia has repeatedly denied any intention to invade Ukraine while pointing to NATO's military activity as a threat to Russian national security.

Just a couple of days after 2,000 US troops land in Europe to "assist" Kiev against Russia, London has also pitched in, sending more than 100 of its special forces advisors to the Ukrainian capital to train and aid the Ukrainian military.