"The news from Lyman is disturbing. At the moment, Lyman is partially surrounded," said Denis Pushilin, the pro-Moscow leader in the breakaway region of Donetsk, on social media.

Two nearby villages were "not fully under our control," he added.

Moscow's forces took weeks to take control of Lyman in the spring.

Its possible imminent recapture by the Ukrainian army comes as Russia prepares to formally annex four occupied regions in a grand Kremlin ceremony later on Friday.

Lyman lies in the north of Donetsk, which Moscow says it will annex despite only controlling part of the region.

Pushilin said Ukrainian forces are "trying to overshadow with all their might this historic event for us."

"Our guys are fighting, we are pulling up reserves and we must hold out but the enemy has also thrown in serious forces."

Kyiv's forces confirmed they were at the doorstep of Lyman, saying they had retaken a nearby village just five kilometres away.

A Ukrainian military unit operating in the area, the 66th mechanised brigade, said on Facebook that it had "liberated the village of Shchurove in Donetsk region."