The order, which also allows US diplomats stationed at the embassy in the Ukrainian capital Kyiv to leave voluntarily, is one of the clearest signs yet that American officials are bracing for an aggressive Russian move in the region.

"Military action by Russia could come at any time," the US Embassy said in a statement on Sunday.

Officials "will not be in a position to evacuate American citizens in such a contingency, so US citizens currently present in Ukraine should plan accordingly".

Ukrainian foreign ministry said it considered the move as "premature and a manifestation of excessive caution".

"In fact, there have been no cardinal changes in the security situation recently: the threat of new waves of Russian aggression has remained constant since 2014 and the buildup of Russian troops near the state border began in April last year," it said.

Tensions in Ukraine have been increasing for months after the Kremlin massed some 100,000 troops near Ukraine's borders, a dramatic build-up the West says is preparation for a war to prevent Ukraine joining the NATO Western security alliance.

The Kremlin has repeatedly denied planning to invade, but the Russian military already tore off a chunk of Ukrainian territory when it seized Crimea and backed separatist forces who took control of large parts of eastern Ukraine eight years ago.

The State Department's announcement comes a day after British authorities said they had information the Russian government was considering a former Ukrainian lawmaker as a potential candidate to head a pro-Russian leadership in Kyiv.

The Russian Foreign ministry dismissed the British allegation as "disinformation," accusing NATO of "escalating tensions" over Ukraine.

Biden has begun considering options for boosting America's military assets in the region, senior administration officials said, after meeting with top national security aides at his Camp David retreat on Saturday.

The New York Times said Biden was mulling plans to send 1000 to 5000 troops to Eastern European countries, with the possibility of increasing the number should tensions flare further.

A senior administration official declined to confirm the numbers on Sunday but said "we are developing plans and we are consulting with allies to determine options moving forward".

The United States has sent military assistance to Ukraine but has so far held back from sending American personnel.

US Secretary of State Antony Blinken has rebuffed calls to immediately impose economic sanctions on Russia, saying on Sunday that doing so would undercut the West's ability to deter potential Russian aggression against Ukraine.

Blinken is due to meet virtually with members of the European Union's Foreign Affairs Committee on Monday.

As US troop deployments were discussed, a separate senior administration official said US economic penalties on Russia would have far-reaching consequences should it drive any further into Ukraine.

Australian Associated Press