The U.S. State Department announced Sunday it was ordering diplomats' family members to leave Ukraine, as U.S. President Joe Biden weighed options for boosting America's military assets in Eastern Europe to counter a buildup of Russian troops.

The order, which also allowed U.S. diplomats stationed at the embassy in the Ukrainian capital Kyiv to leave voluntarily, was 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 U.S. Embassy said in a statement. Officials "will not be in a position to evacuate American citizens in such a contingency, so U.S. citizens currently present in Ukraine should plan accordingly," it added.

Ukrainian foreign ministry said in a statement that 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 buildup the West says is preparation for a war to prevent Ukraine from ever 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.

UK withdraws embassy staff from Ukraine

The UK has begun to withdraw staff from the British embassy in Ukraine amid warnings of a Russian invasion.

Officials say there have been no specific threats to British diplomats, but about half of the staff working in Kyiv will return to the UK.

The US has ordered relatives of its embassy staff to leave, saying an invasion could come "at any time".

Russia has denied plans for military action, but tens of thousands of troops have amassed on the border.

The embassy moves seem to be precautionary, and nothing specific is thought to have occurred in the past 24 hours to have triggered the decisions of the US and UK.

Staff working at the EU embassy will stay in place for now, with EU foreign policy chief Josep Borrell saying he would not "dramatise" the tensions.
-Reuters, BBC