US President Joe Biden has said American citizens should leave Ukraine immediately.
Speaking to NBC News, Mr Biden added that he will not send troops to rescue US nationals from Ukraine if Russia invades.
He added: “It’s not like we’re dealing with a terrorist organisation.
“We’re dealing with one of the largest armies in the world.
“It’s a very different situation and things could go crazy quickly.”
When asked if there was a scenario that could prompt him to send troops to rescue Americans fleeing Ukraine, he 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.”
It comes after the US State Department issued an advisory warning that the US “will not be able to evacuate US citizens in the event of Russian military action anywhere in Ukraine.”
Regular consular services – including helping Americans trying to leave – would be “severely impacted”, it said.