Longer conflict could be in store as Russia eyes Donbas: US official
Ukrainian troops on the frontlines near the city of Kharkiv. (FADEL SENNA AFP)

Russia's refocusing of its military efforts on the Donbas could herald a "longer, more prolonged conflict" as Ukrainian forces put up fierce resistance in the eastern region, a senior US defense official said Thursday.

"It's been fought over now for eight years," the official said of the heavily contested Donbas region of eastern Ukraine.

"The Ukrainians know the territory very, very well," the official said. "They have a lot of forces still there and they're absolutely fighting very hard for that area."

"So just because (the Russians) are going to prioritize it and put more forces there or more energy there doesn't mean it's going to be easy for them," the official said.

"It could be a harbinger of a longer, more prolonged conflict here as the Russians try to gain some leverage, gain some progress, and perhaps gain some chips at the bargaining table."

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.

The US defense official also said the Russia military was continuing to reposition its forces around Kyiv after failing to take the capital.

"They are obviously making decisions to alter their goals and objectives," the official said.

"It remains to be seen whether that will make a bigger difference in their ability to make the Donbas the priority that they say it is going to be."

The official said heavy fighting continued in the southern Ukrainian city of Mariupol and the eastern city of Kharkiv but the Russians have not made any "major gains."

"We believe one of the reasons that they want Mariupol so badly is so that they can move to the north, coming out of Mariupol," the official said.

"We do continue to observe some Russian forces inside Mariupol, obviously the Ukrainians are fighting very, very tough inside the city."