NATO said on Monday it was putting forces on standby and reinforcing eastern Europe with more ships and fighter jets in response to Russia's military build-up at Ukraine's borders.
The move added to a flurry of signals that the West is bracing for an aggressive Russian move against Ukraine, though Moscow denies any plan to invade.
"I welcome allies contributing additional forces to NATO," the Western military alliance's Secretary-General Jens Stoltenberg said in a statement. "NATO will continue to take all necessary measures to protect and defend all allies, including by reinforcing the eastern part of the alliance."
Britain said it was withdrawing some staff and dependants from its embassy in Ukraine in response to "a growing threat from Russia", a day after the United States said it was ordering diplomats' family members to leave.
"Military action by Russia could come at any time," the US Embassy said in a statement. 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," it added.
US diplomats at the embassy in Kyiv were being allowed to leave voluntarily.
The tensions over Ukraine have contributed to a rise in oil, with the latest Russia-U.S. talks on Friday failing to produce any big breakthrough.
Russia is demanding that NATO withdraw a promise to let Ukraine join one day, and that the alliance pull back troops and weaponry from former Communist countries in eastern Europe that joined it after the Cold War.
Washington says those demands are non-starters but it is ready to discuss other ideas on arms control, missile deployments and confidence-building measures.
The United States and the European Union have warned Russia not to invade Ukraine. Denmark said the EU was ready to impose "never-seen-before" economic sanctions and EU foreign ministers meeting in Brussels said they would send a unified warning to Moscow.
An estimated 100,000 Russian troops remain poised within reach of the Ukrainian border. Russia is awaiting a written response to its demands this week.
Ukraine's Foreign Ministry said in a statement that it considered the U.S. embassy 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.
Britain said at the weekend it had information the Russian government was considering a former Ukrainian lawmaker as a potential candidate to head a pro-Russian puppet leadership in Kyiv.
The Russian Foreign Ministry dismissed the British allegation as "disinformation," accusing NATO of escalating tensions over Ukraine.
EU foreign policy chief Josep Borrell said the EU did not plan to withdraw diplomats' families from Ukraine at the moment. German Foreign Minister Annalena Baerbock said Germany would remain present in Ukraine for now but was evaluating the situation continuously.
Latvia warned its citizens not to travel to Ukraine except in cases of "urgent necessity". Lithuania's foreign minister said the West must make sanctions for Russia "unbearable" if it attacked Ukraine.
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