The US State Department said on Sunday that it was ordering diplomats' families to leave Ukraine, as US President Joe Biden considered options for boosting America's military assets in Eastern Europe to counter a Russian force buildup.

The order, which also authorised US diplomats stationed at the embassy in the Ukrainian capital Kyiv to depart voluntarily, was one of the clearest indications yet that American officials are preparing for an aggressive Russian move in the region.

The US Embassy said in a statement that "military action by Russia might come at any time.  Officials will not be able to evacuate American citizens in such a crisis, therefore US citizens currently present in Ukraine should plan accordingly."

The move is "premature and a manifestation of excessive caution," the Ukrainian foreign ministry said in a statement. "In fact, there have been no significant changes in the security situation recently: the threat of new waves of Russian aggression has been ongoing since 2014, and the buildup of Russian forces along the state border began in April last year," it stated.

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Tensions in Ukraine have been rising for months, after The Kremlin deployed some 100,000 Russian troops near Ukraine's borders, which the West claims is in preparation for a war to prevent Ukraine from joining the NATO security alliance.

The Kremlin has repeatedly denied any intention of invading, but the Russian military has already taken a chunk of Ukrainian territory when it seized Crimea and backed separatist troops that took control of vast areas of eastern Ukraine eight years ago.

The announcement by the State Department comes a day after British officials said they had information that the Russian government was considering a former Ukrainian lawmaker as a possible candidate to lead a pro-Russian government in Kyiv.

The British claim was dismissed by the Russian Foreign Ministry as "disinformation," accusing NATO of "escalating tensions" over Ukraine.