NATO announced on Monday that it was putting forces on standby and reinforcing eastern Europe with more ships and fighter jets as the United States and United Kingdom ordered diplomats' families to withdraw from Ukraine amid concerns of a Russian invasion.

"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."

United States said it was ordering diplomats' family members to leave Ukraine while Britain said it was withdrawing some staff and dependents from its embassy in Ukraine in response to "a growing threat from Russia."

"Military action by Russia could come at any time. 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," according to a US embassy statement cited in a Reuters report.  

US diplomats at the embassy in Kyiv were being allowed to leave voluntarily.

Russia has repeatedly denied any plans to invade Ukraine, but has argued that NATO support for the country constitutes a growing threat on Russia's western flank

Russia is also 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.

Washington is not willing to consider these demands but said 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 strict economic sanctions and EU foreign ministers meeting in Brussels said they would send a unified warning to Moscow.

An estimated 100,000 Russian troops have been positioned close to the Ukrainian border as Russia awaits 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, according to a Reuters report.

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

Also read: US military aid lands in Ukraine amid border tension with Russia