President Joe Biden issued a warning Thursday to any Americans who remain in Ukraine as Russia continues to threaten an invasion: Leave.

"American citizens should leave now," Biden said in an interview with NBC News anchor Lester Holt.

“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,” he said.

Holt asked Biden what scenario could prompt him to send troops to rescue Americans fleeing the country. 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," he added.

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