Joe Biden
Joe BidenOfficial White House Photo by Adam Schultz

US President Joe Biden on Thursday issued a warning 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, adding, “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.”

Asked 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.

The interview came as the State Department issued an advisory stating that the US “will not be able to evacuate US 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.”

Biden argued in Thursday’s interview that if Putin is "foolish enough to go in, he’s smart enough not to, in fact, do anything that would negatively impact on American citizens."

Asked if he has ever told Putin that, Biden replied, "Yes."

Tensions between Russia and Ukraine have mounted in recent weeks, after Russia amassed tens of thousands of troops on its border with Ukraine.

Biden warned Ukraine's President recently that there is a "distinct possibility" Russia could take military action against Ukraine in February.

Defense Secretary Lloyd Austin later said that Russian President Vladimir Putin has the capability to launch a military offensive against Ukraine, as he called on Russia to stand down on Ukraine.

Biden has warned Russian President Vladimir Putin that an attack on Ukraine would be met with severe economic consequences for Moscow.

The US President has said he would consider personally sanctioning Putin if Russia invades Ukraine.