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 at 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.
"American citizens should leave now," Mr Biden told NBC News.
"We're dealing with one of the largest armies in the world. It's a very different situation and things could go crazy quickly."
Asked whether there was a scenario that could prompt him to send troops to rescue fleeing Americans, Mr Biden replied: "There's not. That's a world war when Americans and Russia start shooting at one another. We're in a very different world than we've ever been."
World leaders, meanwhile, continued their frenzied diplomacy to defuse the current crisis over Ukraine.
Russia and Ukraine announced late on Thursday that they had failed to reach any breakthrough in a day of talks with French and German officials aimed at ending the separatist conflict in eastern Ukraine.
The current tensions come eight years after Russia annexed Ukraine's southern Crimea peninsula. Since then, Ukraine's military has been locked in a war with Russian-backed rebels in eastern areas near Russia's borders.
Earlier, the UK prime minister said he hoped "strong deterrence" and "patient diplomacy" could find a way through the crisis but the stakes were "very high".
In a joint news conference in Brussels with Nato secretary-general Jens Stoltenberg, Mr Johnson said he did not believe Russia had yet taken a decision on whether to invade Ukraine but the UK's intelligence "remains grim".