Russia could invade Ukraine “at any time” and American citizens should leave immediately, 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.
The US statement prompted countries around the world to issue fresh warnings to nationals in Ukraine.
Non-essential staff have been ordered to leave the US Embassy in Kyiv, the State Department announced in a release. Consular services will be suspended from Sunday, although the US “will maintain a small consular presence” in the western city of Lviv “to handle emergencies”.
UK ambassador to Ukraine Melinda Simmons has tweeted that she and a core team are staying in Kyiv.
Meanwhile, Russia said it has decided to “optimise” its diplomatic staff numbers in Ukraine. A foreign ministry spokeswoman cited fears of “provocations” by Kyiv or other parties.
Attempts to de-escalate tensions through diplomacy are set to continue on Saturday, with both US President Joe Biden and France’s President Emmanuel Macron due to speak to Russia’s Vladimir Putin by phone.
Moscow has accused Western countries of stirring up hysteria.
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.