Russia may be repositioning some of its forces around the Ukrainian capital Kyiv to send them to the eastern Donbas region, where Ukrainian forces have been putting up fierce resistance, the Pentagon said Thursday.

Pentagon spokesman John Kirby also said that Defense Secretary Lloyd Austin has decided to keep elements of the 82nd Airborne Division in Europe for the time being along with an aircraft carrier strike group in the Mediterranean.

Russia has moved a "small number" -- perhaps 20 percent -- of its troops from around Kyiv after failing to capture the city, which continues to be targeted by Russian airstrikes, Kirby said.

"It's not exactly clear where they're going to go, for how long, and for what purpose," he said, "but we don't see any indication that they're going to be sent home."

The Pentagon spokesman said the "best assessment" is that the troops are "going to be repositioned, probably into Belarus, to be refit and resupplied and used elsewhere in Ukraine."

He noted that Russia has said it plans to "reprioritize" its operations in the Donbas area. "That could be one destination," he said.

Sergei Rudskoi, a senior Russian military leader, said last week that the first phase of the military campaign in Ukraine was over and troops would now focus on the "main goal -- the liberation of Donbas," which is already partly held by Russian proxies.

Ukrainian troops on the frontlines near the city of Kharkiv
Ukrainian troops on the frontlines near the city of Kharkiv AFP / FADEL SENNA

Kirby noted that the Donbas region has been fought over for eight years and the Ukrainian armed forces have been "very active" in the area.

"This could drag on for a while," he said. "It might not just be a matter of days and weeks, it could be much longer than that. It's really difficult to know."

The Pentagon spokesman said Austin had decided to keep elements of the 82nd Airborne Division in Europe and the USS Harry S. Truman aircraft carrier strike group in the Mediterranean.

"They're not going anywhere, anytime in the immediate future," Kirby said. "We want to be able to monitor the situation on the ground and make the best and most flexible decisions in real time."

Kirby said the United States has also received indications that some Russian forces have left the area around the accident-hit Chernobyl nuclear plant.

"We gather that they are leaving to the north to go back again towards Belarus," he said, adding that it was "not necessarily being done because of health hazards or some sort of emergency or a crisis at Chernobyl."

He said he was not able to "definitively" say who was in control now of Chernobyl, the site of a 1986 nuclear accident.