The United States National Security Adviser Jake Sullivan warned that the 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 (President) 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," he added.

Sullivan said the U.S. 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."

"The risk is high enough and the threat is now immediate enough that prudence demands that it is the time to leave now," Sullivan said.

"We are not saying that a decision has been taken by President Putin," Sullivan said.

"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," he urged.

Meanwhile, the U.S. is planning to send 3,000 additional troops from 82nd Airborne Division to Poland, who will leave in the next couple of days.

These 3,000 airborne troops based at Fort Bragg in North Carolina were already placed on alert in late January at the request of President Joe Biden.

They will now leave the base in the coming days and are expected to arrive in Poland early next week, a senior defense official said.