"They're repositioning, they're refocusing on the Donbas," Pentagon spokesman John Kirby told reporters.

Kirby said a convoy of vehicles had been observed heading for Izyum but "it's not clear to us how many vehicles are in this convoy and what exactly they're bringing.

"It does seem to be a mix of personnel-carrying vehicles as well as armored vehicles and maybe some artillery," he said.

The Pentagon spokesman underscored that the Ukrainian military has been fighting Moscow-backed separatists in the Donbas region since 2014.

"The Ukrainians have been for eight years -- and still are -- fighting over that," he said. "And they show no signs of being willing to give that territory up."

A senior US defense official said earlier Monday that the United States does not believe a "new offensive" in the Donbas region has begun yet.

"We still assess that while there is fighting going on they (the Russians) are working to reinforce their capabilities and to add to it," the official said.

The official also confirmed that Russian general Alexander Dvornikov, a veteran of Russia's military intervention in Syria, had been named the overall commander of Russian forces in Ukraine.

"Our assessment is that the Russians have in fact appointed him to be the overall commander," the official said.

"But it remains to be seen, given the significant logistics and sustainment challenges, the operational maneuver challenges, the integration of fires challenges, the morale, the leadership, it certainly remains to be seen what sort of an effect he's going to be able to have," the official said.

Pentagon spokesman Kirby said he was unable to confirm a claim by Ukrainian President Volodymyr Zelensky that "tens of thousands" of people may have been killed in the southern city of Mariupol.

"I don't think anybody's really going to know until Ukrainian authorities are able to get in there and look and see," he said. "It's still being fought over.

"But if you just look at the imagery, and you see how much the Russians have pounded Mariupol from the air, it's inconceivable to imagine that there aren't going to be civilian casualties, and that it could be a significant number," he said.