US President Joe Biden has called on all American citizens remaining in Ukraine to leave the country immediately, citing increased threats of Russian military action.
Mr Biden said he would not send troops to rescue Americans if Moscow invades Ukraine.
He warned that “things could go crazy quickly” in the region.
Russia has repeatedly denied any plans to invade Ukraine despite massing more than 100,000 troops near the border but it has just begun massive military drills with neighbouring Belarus, and Ukraine has accused Russia of blocking its access to the sea.
The Kremlin says it wants to enforce “red lines” to make sure that its former Soviet neighbour does not join Nato.
UK Prime Minister Boris Johnson said on Thursday that Europe faced its biggest security crisis in decades amid the tensions.
The US State Department urged Americans in Ukraine to leave immediately.