Ukrainian presidential advisor Mykhailo Podolyak said Russian forces were "continuously attacking" the encircled Azovstal steel plant in the besieged city of Mariupol, CNN reported.
The Azovstal steel mill, where the defenders are holed up is the last corner of resistance in the city, otherwise occupied by the Russians.
Ukraine has urged Russia to allow the evacuation of civilians holed up at the giant plant. It also has pushed Russia to conduct talks on a safe exit for the Ukrainian defenders of the plant, but Moscow has stonewalled the demands.
Mariupol has been the focus of fierce fighting since the start of the war due to its location on the Sea of Azov. Its capture would deprive Ukraine of a vital port, free up Russian troops to fight elsewhere, and allow Moscow to establish a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014.
More than 1,00,000 people down from a prewar population of about 430,000 are believed to remain in Mariupol with scant food, water or heat. Ukrainian authorities estimate that over 20,000 civilians have been killed. Satellite images released this week showed what appeared to be mass graves dug in towns to the west and east of Mariupol.