Updated February 11, 2022 at 2:36 PM ET

The U.S. isn't planning a military operation to help its citizens leave Ukraine, President Biden says, and they should do so on their own. "American citizens should leave now," Biden said as Russia amasses a military force around its neighbor.

Biden made the remarks in an interview with NBC News' Lester Holt, who asked him what kind of scenarios would prompt a U.S. rescue mission, if Russia invaded.

"There's not [one]," Biden said. "That's a world war — when Americans and Russians start shooting at one another, we're in a very different world than we've ever been in."

A military-run evacuation would be extremely complicated, Biden said, including the challenge of locating U.S. citizens in Ukraine.

Referring to Russian President Vladimir Putin, Biden added, "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."

"He knows that," Biden added later. "What I've asked is, American citizens should leave — should leave now."

Biden contrasted this situation with the Taliban takeover of Afghanistan, which saw the U.S. mount a dayslong effort to fly U.S. citizens and some Afghans out of the country.

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

The U.S. State Department issued a "Level 4: Do Not Travel" advisory for Ukraine on Thursday, saying U.S. citizens in Ukraine "should depart now via commercial or private means," in large part because of the high threat of Russian military action (the advisory also mentions Ukraine's "very high level" of COVID-19 cases).

"If remaining in Ukraine, exercise increased caution due to crime, civil unrest, and potential combat operations should Russia take military action," the State Department's advisory says.

The European Union's diplomatic service is asking all its nonessential staff in Ukraine to leave. The Netherlands, the U.K. and Israel are among countries asking their citizens to leave Ukraine immediately and asking those planning to travel there to refrain from doing so.

Russia has positioned around 140,000 troops near its border with Ukraine and in Crimea, according to Russian media. Putin's military is currently conducting what's being called its largest exercise since the Cold War, in joint maneuvers with Belarus. Russia has also sent warships into the Black Sea for naval exercises, raising fears of a potential blockade or invasion.

Because of the tensions, Biden last week deployed 2,000 U.S. troops to Europe to bolster its NATO allies. He also ordered another 1,000 troops who are based in Germany to move to Romania.

If Russia defies international pressure and mounts an invasion, Belarus might be part of the plan. A recent analysis by the Center for Strategic and International Studies found that Russia's military could attempt "to outflank Ukrainian defenses around Kiev by approaching through Belarus."

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