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 demoralise and disorientate the international community," foreign ministry spokesman Alexander Lukashevich said.

US 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.

She also accused Russia of sending hundreds of pieces of military equipment to the rebel-controlled zone since December, including tanks and rocket systems.

Russia's deputy defence minister Anatoly Antonov also denied the allegation.

He also rejected a claim by the head of US Army forces in Europe, Lieutenant General Ben Hodges, who said 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 caustically at a briefing, according to the RIA Novosti news agency.

NATO deputy secretary-general Alexander Vershbow on Thursday, speaking at a conference in the Latvian capital Riga, said: "Russian leaders are less and less able to conceal the fact that Russian soldiers are fighting and dying in large numbers in Eastern Ukraine."

He said that "Putin's aim seems to be to turn Ukraine into a failed state and to suppress and discredit alternative voices in Russia so as to prevent a Russian Maidan."

Moscow denies any involvement in the conflict, which the UN 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.

Both Kiev and the separatist rebels have said they are withdrawing heavy weapons from the frontline as per a February 15 ceasefire agreement which is being closely watched by Western countries after repeated violations.

Western leaders including US President Barack Obama on Tuesday called for a "strong reaction" from the international community to any major violation of the ceasefire, hinting at the possibility of further sanctions on Russia.

The UN Security Council will meet Friday to take stock of the latest efforts to prop up the EU-mediated ceasefire signed in Minsk which aims to end the 10-month conflict, diplomats said.