"American citizens should leave now," Biden said in an upcoming interview that was taped Thursday with NBC News' Lester Holt.
Referring to Russia, which has amassed troops on its border with Ukraine, Biden said, "It's not like we're dealing with a terrorist organization. We're dealing with one of the largest armies in the world. It's a very different situation and things could go crazy quickly."
Biden said during his NBC interview that "there's not" a situation that could prompt him to send US troops to rescue Americans attempting to exit Ukraine, adding, "That's a world war when Americans and Russia start shooting at one another."
If Russian President Vladimir Putin is "foolish enough to go in, he's smart enough not to ... do anything that would negatively impact on American citizens," Biden added.
The White House has approved a plan for the nearly 2,000 US troops in Poland to help Americans who may try to evacuate Ukraine if Russia invades, according to two US officials familiar with the matter.
The US forces are not currently authorized to enter Ukraine itself if a war breaks out, and there are no plans for them to conduct a noncombatant evacuation operation akin to the US operation in Afghanistan last summer. Instead, the plan as it now stands is that the troops, who are from the 82nd Airborne Division, will begin setting up processing areas and temporary shelters inside Poland near Ukraine's border where Americans fleeing the country could go for help while in transit. The facilities have not yet been stood up, one defense official said, but will start to be as more US troops arrive in Poland.
The US State Department on Thursday repeated its warning saying that Americans should not travel to Ukraine "due to the increased threats of Russian military action" and called on those in the country to depart immediately.
The advisory told American citizens in Ukraine to "be aware that the US government will not be able to evacuate US citizens in the event of Russian military action anywhere in Ukraine."
In late January, the State Department authorized the departure of non-emergency personnel from the US Embassy in Kyiv and ordered family members to depart the country.
The US has estimated that Russia has more than 100,000 troops near the Ukraine border, with thousands added just this week, according to an administration official.
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