Biden urges Americans to immediately leave Ukraine

US President Joe Biden urged Americans on Thursday to immediately leave Ukraine, as Russia's live-fire drills and build-up of troops around the ex-Soviet state deepened fears of an invasion.

Washington-Moscow tensions are at their highest since the Cold War, with some US estimates saying some 130,000 Russian soldiers are grouped in dozens of combat brigades near the border with Ukraine.

"American citizens should leave now," Biden said in a pre-taped interview with NBC News.

"We're dealing with one of the largest armies in the world. It's a very different situation and things could go crazy quickly."

Biden reiterated that under no circumstances would he send US troops to Ukraine, even to rescue Americans in case of a Russian invasion.

"That's a world war. When Americans and Russians start shooting one another, we're in a very different world," he said.

Biden's remarks were released hours after Russia rolled its tanks across Belarus for live-fire drills that drew an ominous warning from NATO and added urgency to Western efforts to avert war on the continent, reports AFP.

NATO said Russia's deployment of missiles, heavy armor and machine-gun toting soldiers marked a "dangerous moment" for Europe some three decades after the Soviet Union's collapse.

Western leaders have been shuttling to Moscow in an effort to keep the lines of communication open, giving Russia a chance to air its grievances about NATO's expansion into eastern Europe and ex-Soviet states.

But they have also sought to project their resolve in the face of what they is Russian escalation of an already-tense situation.

"Russia should not underestimate our unity and determination as a partner in the EU and as an ally in NATO," German Chancellor Olaf Scholz warned.

In a bid to "reduce chances of miscalculation" during the drills, US and Belarusian defense chiefs held rare telephone talks, the Pentagon said Thursday.

Russia has also sent six warships through the Bosphorus for naval drills on the Black Sea and the neighboring Sea of Azov. Kyiv condemned their presence as an "unprecedented" attempt to cut off Ukraine from both seas.

Moscow and Minsk have not disclosed how many troops are participating, but the United States has said around 30,000 soldiers were being dispatched to Belarus from locations including Russia's Far East.