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

Several other nations have asked their citizens to leave Ukraine.

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 was quoted as saying by BBC.

Joe Biden earlier cited increased risk of Russian military invasion as he advised American citizens to leave Ukraine.

Biden mentioned that he would not send troops to rescue Americans if Moscow invades Ukraine.

He warned that "things could go crazy quickly" in the region, reports BBC.

"American citizens should leave now," Biden said in an interview with NBC News anchor Lester Holt.

“It’s not like we’re dealing with a terrorist organization. We’re dealing with one of the largest armies in the world. It’s a very different situation, and things could go crazy quickly,” the US President said.

Russia has sent more than 100,000 troops near the border with Ukraine.

However, Russia has repeatedly rejected any plans of invading Ukraine.