Ukraine has said it will not agree to any ceasefire deal that would involve handing over territory to Russia, as Moscow intensified its attack in the eastern Donbas region on Sunday.
Controlling Mariupol gives Russia command of a land route linking the Crimean peninsula, which Moscow annexed in 2014, with mainland Russia and parts of eastern Ukraine held by pro-Russia separatists.
Shelling and missile strikes hit Kharkiv in northern Ukraine and Mykolaiv and Zaporizhzhia in the south, while eight civilians were killed on the eastern front in the Donbas, Ukrainian officials said.
Severodonetsk and Lysychansk, its twin city on the other side of the Siverskiy Donets River, form the eastern part of a Ukrainian-held pocket that Russia has been trying to overrun since mid-April after failing to capture Kyiv and shifting its focus to the east and south of the country.
The Russian army has flattened the Black Sea port of Mariupol during its siege and subjected Ukrainian troops and towns in the east to relentless ground and artillery attacks.
AFP, Associated Press and Reuters contributed to this report
