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.

Separately on Thursday, the State Department issued an advisory warning that the U.S. “will not be able to evacuate U.S. citizens in the event of Russian military action anywhere in Ukraine.” It warned that regular consulate service — including aiding citizens trying to leave the country — would be “severely impacted.”