US Vice President Joe Biden has encouraged Americans to evacuate Ukraine immediately, warning that "things could go crazy quickly" in the event of a Russian invasion.

Mr. Biden warned Thursday in an interview with NBC News that there was a risk of a catastrophic battle with Moscow if US and Russian military engaged on the ground and that "American citizens should leave now."

"We're up against one of the world's largest armies. It's a very different circumstance, and things might quickly spiral out of control "According to the president.

When asked whether he could envision a scenario in which he would send troops to rescue fleeing Americans, Biden responded: "There isn't. When the United States and Russia begin shooting at one another, that is a declaration of war. We are living in a planet unlike any other in human history."

His remarks reinforced an earlier statement he made this week in which he stated that it would be "wise" for Americans to leave Ukraine.

The State Department issued a similar warning on Thursday, asserting that the US "will not be able to evacuate US citizens in the event of Russian military action anywhere in Ukraine."

Russia, which has over 100,000 troops stationed near Ukraine's borders, dismisses Western charges plotting an invasion of its former Soviet neighbor. In contrast, it has indicated that it may take unspecified "military-technical" action if specific demands are not satisfied.

It held military drills in Belarus and the Black Sea on Thursday, while Ukraine also held exercises.

Britain warned on Thursday that the West's confrontation with Moscow was approaching its "most dangerous moment."

Russia's rising threats of military action come as western countries seek to broker a diplomatic solution to the crisis.

Liz Truss was criticized Thursday during a joint press conference with her counterpart for failing to achieve a breakthrough during a high-stakes visit to Moscow, the first by a British foreign secretary in more than four years.

Sergey Lavrov, Russia's seasoned foreign minister, described their conversation as akin to "talking to a deaf person" following their meeting.

"It's as if they're listening but not hearing," said Mr. Lavrov, Russia's top diplomat since 2004.