The United States' senior envoy to Europe alleged Wednesday that Russia had deployed "thousands and thousands" of troops to neighboring Ukraine.

Speaking to a congressional foreign affairs committee, Assistant Secretary of State Victoria Nuland also told US lawmakers that Russia was flooding eastern Ukraine with military hardware.

"Since December, Russia has transferred hundreds of pieces of military equipment, including tanks, armored vehicles, rocket systems, heavy artillery," Nuland said.

"The Russian military has its own robust command structure in eastern Ukraine... they are funding this war, they are fueling it and commanding and controlling it."

Nuland was not able to say exactly how many Russian troops were in eastern Ukraine -- where pro-Russian rebels are battling loyalist security forces -- but estimated the number to be "in the thousands and thousands."

On Tuesday, Western leaders, including US President Barack Obama, called for a "strong reaction" from the international community to any major violation of a ceasefire in eastern Ukraine as they sought to further pressure Russia over the conflict.

The leaders did not specify what a "major" break from the accord might be, nor what response it would provoke, but hinted at the possibility of further sanctions on Moscow.

But, in her testimony to the House Foreign Affairs Committee, Nuland suggested that Washington and its partners were ready to further tighten economic sanctions on Russia.

She also accused the Kremlin of violating the terms of a ceasefire deal agreed in Minsk.

"We have already begun consultations with our European partners on further sanctions pressure should Russia continue fueling the fire in the east or other parts of Ukraine, fail to implement Minsk or grab more land as we've seen in Debaltseve," she said.

Moscow denies any involvement in the conflict, which has cost some 6,000 lives over the past year, although it admits some Russian troops may have volunteered to fight with the rebels while on leave from their regular units.