WASHINGTON >> President Joe Biden today warned Americans to leave Ukraine, saying that U.S. troops would not be dispatched to retrieve them should Russia invade.

“American citizens should leave, should leave now,” Biden said in an interview with NBC’s Lester Holt that aired this evening, adding that there was no scenario that could prompt him to send troops to rescue Americans. “We’re dealing with one of the largest armies in the world. This is a very different situation, and things could go crazy quickly.”

“That’s a world war when Americans and Russia start shooting at one another,” Biden added. “We’re in a very different world than we’ve ever been in.”

Biden’s comments followed a string of increasingly urgent warnings for U.S. citizens to leave Ukraine as thousands of Russian troops have amassed on its borders.

TheState Department said today that “military action may commence at any time and without warning,” as it reissued an advisory urging Americans not to travel to Ukraine with its starkest language yet.

A military incursion would also “severely impact” the U.S. Embassy’s ability to help Americans leave Ukraine, the department said. In October, Ned Price, a State Department spokesperson, estimated that about 6,600 U.S. citizens were living in Ukraine.

Biden had previously made clear that he had no intention of sending U.S. troops to defend Ukraine, stating pointedly in early December that the military option was “not on the table,” and this week, he warned Americans that “it would be wise” to leave the country.

But White House press secretary Jen Psaki avoided directly answering a question Wednesday on whether the United States would assist Americans “stranded” after an invasion.

“The United States does not typically do mass evacuations,” Psaki said in a briefing. “There are a range of means that individuals and Americans can depart from Ukraine, and we’ve been encouraging them to do exactly that.”

Satellite images from Wednesday and today showed new Russian forces and equipment still arriving on three sides of Ukraine.

Senior Biden administration officials told lawmakers this month that President Vladimir Putin of Russia had assembled everything he would need to undertake an invasion that could take an enormous human toll, including the potential deaths of 25,000 to 50,000 civilians.

Among the most aggressive actions Putin could take if he invades is to quickly surround and capture Kyiv, the capital. But officials have stressed that U.S. intelligence analysts still do not think Putin has yet decided whether to invade.

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

Asked by Holt if he had told Putin whether that was a “line that they can’t cross,” the president was adamant.

“I didn’t have to tell him that,” Biden said. “I’ve spoken about that. He knows that.”

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This article originally appeared in The New York Times.