Asked if there was a scenario in which he would deploy troops to help evacuate Americans, Biden said: “There’s not. That’s a world war when Americans and Russia start shooting at one another.”

The president repeated a warning for Americans still in Ukraine to leave now, adding: “We’re in a very different world than we’ve ever been.”

More than 100,000 Russian troops have massed along the Ukrainian border, stoking fears that an incursion could come at any time. Russian President Vladimir Putin has said he has no plans to invade Ukraine, but his forces continue to conduct military drills along the border. NATO allies have responded by shoring up military support along the eastern flank.

Biden said he has told Putin that any action that would affect Americans would escalate the already tense stand-off.

“‘I’m hoping that, if in fact he’s foolish enough to go in, he’s smart enough not to … do anything that would negatively impact on American citizens,” Biden said.

Asked if he’s told Putin that Americans would be a red line, Biden responded: “I didn’t have to tell him that. I’ve spoken about that. He knows that,” he said.

‘Things could go crazy’: Biden urges Americans to leave Ukraine amid threat of Russian invasion (