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.”
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PM meets British troops in Poland
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.”
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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 to leave – would be “severely impacted”, it said.
Russia’s president Vladimir Putin has denied plans to attack Ukraine, but many thousands of Russian troops are on the country’s border.
Britain, the US, France and others have been trying to find a diplomatic solution to the crisis in recent days, but little progress appears to have been made.
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‘The most dangerous moment for decades’
Troops from NATO countries have been sent to eastern European and Baltic states in case Russia attacks, while weapons have been sent to the Ukrainian armed forces.
Mr Biden said that if Mr Putin is “foolish enough to go in, he’s smart enough not to, in fact, do anything that would negatively impact on American citizens.”
When asked if he had told Mr Putin that “Americans will be a line that (Russia) can’t cross”, Mr Biden said: “I didn’t have to tell him that – I’ve spoken about that, he knows that.”