US President Joe Biden is not sending troops to rescue fleeing Americans, saying: "That's a world war when Americans and Russia start shooting at one another."
US President Joe Biden has urged Americans to leave Ukraine immediately, as his top diplomat said that a Russian invasion could come "any time" – including during the Winter Olympics, which end in nine days.
"American citizens should leave, should leave now," Biden told NBC News on Thursday. "We're dealing with one of the largest armies in the world. This is a very different situation and things could go crazy quickly."
Biden also said he would not send troops to get Americans out of Ukraine.
"That's a world war when Americans and Russia start shooting at one another," he said.
On the other side of the world in Melbourne, his Secretary of State Antony Blinken huddled with Asia-Pacific allies, stressing that Russian President Vladimir Putin may be days, or even hours away from launching a war in continental Europe.
"We're in a window when an invasion could begin at any time, and to be clear that includes during the Olympics," Blinken said, brushing aside suggestions that the veteran Russian leader would wait until the Beijing Games end on January 20 to avoid upstaging his Chinese allies.
"Simply put, we continue to see very troubling signs of Russian escalation," Blinken said.
Observers have described the gathering of Russian forces on three of Ukraine's flanks as the largest show of force since the Soviet Army marched on Berlin at the end of World War II.
Some US estimates put the number of Russian soldiers at 130,000, grouped in dozens of combat brigades.
On Thursday, Russia rolled its tanks across Belarus for live-fire drills. Washington has said that around 30,000 soldiers were involved, coming from locations that included Russia's Far East.
Russia has also sent six warships through the Bosphorus for naval drills on the Black Sea and the neighbouring Sea of Azov.
Kiev condemned their presence as an "unprecedented" attempt to cut off Ukraine from both seas.
Kiev has launched its own military drills expected to mirror Russia's games, but officials have said little about them out of apparent fear of escalating tensions.
With the drumbeat of war growing louder, the already intense push by European leaders to find a diplomatic solution gained a new sense of urgency.
Russia is seeking written guarantees that NATO will withdraw its presence from Eastern Europe and never expand into Ukraine.
The United States and its European allies have officially rejected Russia's demands.
"Difficult talks" between German, Russian, Ukrainian and French representatives broke late Thursday, with the quartet agreeing to meet again in March.