The US has warned that Russia has the troops in place to invade Ukraine "at any time" and American citizens should leave within the next 48 hours.
The White House says an invasion could start with aerial bombing that would make departures difficult and endanger civilians.
A host of other countries have also urged their nationals to leave Ukraine.
Moscow has repeatedly denied any plans to invade Ukraine despite massing more than 100,000 troops near the border.
Russia's foreign ministry has accused Western countries of spreading false information.
US National Security Adviser Jake Sullivan says 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.
[Source: BBC]
