President Joe Biden is urging Americans to leave Ukraine immediately, as his top diplomat issued a fresh warning that a Russian invasion of Ukraine “could begin at any time.”

In an interview recorded Thursday with NBC News, Biden said American citizens in Ukraine “should leave now.”

“We're dealing with one of the largest armies in the world,” the president said. “This is a very different situation, and things could go crazy quickly.”

Biden said the U.S. has no plans in place to evacuate American citizens from Ukraine during a Russian invasion. 

“That's a world war when Americans and Russians start shooting at one another,” he said.

Biden also suggested it would be extremely difficult for U.S. forces to locate American citizens in order to evacuate them. 

“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 American citizens,” Biden said of Russian President Vladimir Putin.

Biden said Putin knows that harming Americans is a red line, but he wasn’t clear about whether he directly communicated that to the Russian leader, saying: “I didn’t have to tell him that. I’ve spoken about that. He knows that.”

Biden held a call Friday morning with a number of foreign leaders to discuss the ongoing crisis in Ukraine, including the United Kingdom's Boris Johnson, Canada's Justin Trudeau, France's Emmanuel Macron, Germany's Olaf Scholz, European Commission president Ursula von der Leyen and NATO secretary-general Jens Stoltenberg.

Johnson warned allies that he fears for the "security of Europe" and the U.K. updated its travel policy on Friday urging all of its citizens to "leave now while commercial means are still available."

The State Department issued a Level 4 travel advisory for Ukraine on Thursday, warning Americans not to travel there and urging those in the eastern European country now to leave “due to the increased threats of Russian military action.”

Last month, the State Department authorized the departure of nonemergency personnel from the U.S. Embassy in Kyiv and ordered family members of embassy employees to leave the country.

Russia has amassed more than 100,000 troops near Ukraine’s border, and the Pentagon believes Russia has at least 70% of the forces in place to execute a full-scale invasion of Ukraine.

Biden told NBC that Putin “has to know that if he does” invade Ukraine “the entire circumstance for Russia changes worldwide, changes overnight. The cost to Russia, both in terms of reputational costs and economic costs, would be profound.”

Secretary of State Antony Blinken said Friday the United States continues “to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border.”

“We're in a window when an invasion could begin at any time — and to be clear, that includes during the Olympics," Blinken told reporters Friday at a meeting with foreign ministers from Australia, India and Japan in Melbourne, Australia. 

"We're continuing to draw down our embassy," he added. "We will continue that process. And we've also been very clear that any American citizens who remain in Ukraine should leave now.”