"They have about a third left of the forces that they had arrayed against Kyiv," the official said on grounds of anonymity.

"We do begin to see them consolidating in Belarus. What we continue to believe is that they're going to be refit, resupplied, perhaps maybe even reinforced with additional manpower, and then sent back into Ukraine to continue fighting elsewhere," the official said.

The redeployment comes after Ukraine forces delivered a strong pushback against the Russians, who invaded Ukraine on February 24 with the apparent intent of quickly capturing Kyiv, the capital, and replacing the government.

Western military analysts have called the failed Kyiv siege a significant defeat for the Russians, but Moscow has suggested it wants to focus its military efforts on the country's southeast Donbas region, where they have joined hands with pro-Russia secessionist forces.

The Pentagon official said the withdrawn troops had not yet shown signs of moving to Donbas. "We haven't seen them begin to move," the official said.

Nevertheless, the official said, the Pentagon believes that the Russians are now going to "become more aggressive" in the Donbas region.

The official did not deny reports that Western allies of Ukraine are planning to supply it with tanks made in the former Soviet Union to continue their counterattack on Russian forces.

"We continue to discuss and talk with allies and partners about security assistance for Ukraine," the official said.

"But these decisions about what a nation provides to Ukraine are national decisions that they have to make for themselves."

Nine weeks into the war, Washington has stressed that it was only providing "defensive" arms to Ukraine, like precision weapons to destroy Russian armored vehicles.

Some 14 other countries are providing what are generally more offensive-use weapons, the official said.