Russian officials on Thursday dismissed a claim by the United States that Moscow has sent "thousands" of troops to fight alongside pro-Russian separatists in Ukraine.

"These figures, which are plucked out of the air, of course demoralize and disorientate the international community," foreign ministry spokesman Alexander Lukashevich said.

U.S. Assistant Secretary of State Victoria Nuland on Wednesday told a congressional foreign affairs committee that Russia had deployed "thousands and thousands" of troops to Ukraine, although she said she could not give a precise number.

Russia's deputy defense minister Anatoly Antonov also denied the allegation, as well as that by the head of U.S. Army forces in Europe, Lieutenant General Ben Hodges, who said that there were 12,000 troops in Ukraine,.

"I know about the statements by Victoria Nuland and the general about those thousands of Russian servicemen allegedly in Ukraine. But why 12,000. Why are they thinking small? Why didn't they say 20,000 why didn't they say 25,000?" Antonov said at a briefing, according to the RIA Novosti news agency.

Moscow denies any involvement in the conflict, which the U.N. says has left about 6,000 dead in the past year, although it admits some Russian troops may have volunteered to fight with the rebels while on leave from their regular units.