Ukraine's military said on Tuesday about 300 Russian soldiers were inside the embattled town of Pokrovsk, and that Moscow had intensified efforts to get more troops in over the past few days, using dense fog for cover.

"Their goal remains unchanged - to reach the northern borders of Pokrovsk and then attempt to encircle the agglomeration," Ukraine's 7th paratrooper corps said on Facebook.

Fierce battles around Pokrovsk, once a key Ukrainian road and rail hub but now a ruined town, have been raging for months as Moscow grinds forward slowly in eastern Ukraine.