Ukrainian and Russian troops engaged in heavy fighting in and around Severodonetsk.
Russia's army said Saturday that some Ukrainian military units were withdrawing from the key city of Severodonetsk in eastern Ukraine.
"Some units of the Ukrainian army, having suffered critical losses during fighting for Severodonetsk, are pulling out towards Lysychansk," Severodonetsk's twin city, which sits just across a river, the defence ministry said in a statement.
It added however that some Ukrainian fighters remained in the city.
"The Ukrainian authorities, having realised that it was impossible to resist further and to hold on to the industrial zone of Severodonetsk, ordered a mixed tactical unit to mine the tanks containing nitrate and nitric acid in the Azot factory," it said.
Russia's army said this group still in the city included "survivors" of the 79th Air Assault Brigade and members of the Ukrainian Territorial Defence Forces.
It claimed the Ukrainian army sought to contaminate the area and "delay" the Russian military operation.
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