United States President Joe Biden on Thursday told American citizens living in Ukraine to leave amid growing fears of a Russian invasion on the country, reported NBC.

“American citizens should leave now,” Biden told NBC in an interview.

Since early this year, Russia has amassed close to 1.3 lakh troops at the Ukrainian border, according to the New York Times. The two countries have been engaged in a conflict since 2014 when Russia annexed Ukraine’s Crimean Peninsula and backed separatist rebellions in the country’s eastern regions of Donetsk and Luhansk.

US military and intelligence assessment suggests that the Russian military could launch a full-scale invasion of Ukraine with tanks that potentially could reach its Capital, Kyiv, within 48 hours of the assault, according to NBC.

In Thursday’s interview, Biden said that the US was not dealing with a terrorist organisation, but one of the largest armies in the world.

“It’s a very different situation, and things could go crazy quickly,” he said.

When asked by host Lester Holt about the circumstances under which Biden would send troops to rescue American citizens from Ukraine, the president replied: “There’s not [any circumstances]. That’s a world war when Americans and Russia start shooting at one another.”

He added: “We’re in a very different world than we’ve ever been.”

Earlier this month, the US deployed 3,000 troops in Europe. Pentagon spokesperson John Kirby had said that troops would not be fighting in Ukraine, but were being sent to support members of the North Atlantic Treaty Organization, or NATO.

Meanwhile, the US State Department has issued an advisory warning that it “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.”

Escalation along the Ukraine border

Biden’s remarks in the NBC interview came hours after Russia sent its tanks and troops across Belarus – which shares it borders with Russia and Ukraine – for live fire drills, reported AFP. While Russia and Belarus have not disclosed the number of troops participating in the drills, the US has said about 30,000 soldiers were being dispatched.

Russia has also sent six warships through the Strait of Istanbul, for naval drills. The strait is an important waterway, located in northwest Turkey, that forms part of the continental boundary between Asia and Europe.

Kyiv has condemned the presence the ships as an “unprecedented” attempt to cut off Ukraine from both the Black Sea and the Sea of Azov. Ukraine’s President Volodymyr Zelensky has said the presence of troops was “psychological pressure” from Russia.

The North Atlantic Treaty Organization, an intergovernmental military alliance that has 27 European countries as its members, said Russia’s deployment of armaments and soldiers marked a “dangerous moment” for Europe.

“Russia should not underestimate our unity and determination as a partner in the EU [European Union] and as an ally in NATO,” German Chancellor Olaf Scholz said.

Meanwhile, Russia’s defense ministry has said that the exercises in Belarus are aimed at “suppressing and repelling external aggression” and promised the troops will return after the drill.