“This is a very brutal battle, very tough, perhaps one of the most difficult throughout this war,” Ukrainian President Volodymyr Zelensky said in a video statement on Wednesday.

“Sievierodonetsk remains the epicentre of the encounter in Donbas … Largely, that is where the fate of our Donbas is being decided now,” he added.

Ukrainian fighters pulled back to the city’s outskirts on Wednesday but have vowed to fight there for as long as possible.

“The enemy fired on our units with mortars, artillery and multiple rocket launchers,” the Ukraine general staff said on Thursday.

“It fired on civilian infrastructure in the settlements of Sievierodonetsk, Lysychansk, Privillya, Ustynivka, Horske and Katerynivka.”

Russia denies targeting civilians.

Ukrainian forces still control all of Sievierodonetsk’s smaller twin city Lysychansk on the west bank of the Siverskyi Donets River, but Russian forces were destroying residential buildings there, Gaidai said.

The situation on the ground in either city could not be independently verified.

Kyiv’s ambassador to the United States told CNN that Ukrainian troops were vastly outnumbered in Luhansk and Donetsk, which collectively form the Donbas, a largely Russian-speaking region.

But “as we already saw in the battle for Kyiv, we can lose something temporarily. Of course, we’re trying to minimise that because we know what (can) happen (when) Russians control territories, but we will get it back,” Oksana Markarova said.

Gaidai said Russia now controlled more than 98 per cent of Luhansk.

Meanwhile in Russian-held Mariupol, the human cost of the war in Ukraine mounted as workers pulled up to 100 bodies from each smashed building in the devastated city.

Many buildings in Mariupol contain 50 to 100 bodies each, according to a mayoral aide in the Russian-held port city in the south.

Petro Andryushchenko said on the Telegram app that the bodies are being taken in an “endless caravan of death” to a morgue, landfills and other places.

At least 21,000 Mariupol civilians were killed during the weeks-long Russian siege, Ukrainian authorities have estimated.

Moscow says it is engaged in a “special military operation” to disarm and “denazify” its neighbour. Ukraine and its allies say Moscow has launched an unprovoked war of aggression, killing thousands of civilians and flattening cities.

Russia’s continuing encroachment in Ukraine could open up the possibility of a negotiated settlement between the two nations more than three months into the war, analysts said.

Russian President Vladimir Putin “has the option of declaring his objectives met at more or less any time in order to consolidate Russia’s territorial gains,” said Keir Giles, a Russia expert at the London think tank Chatham House.

At that point, Giles said, Western leaders may “pressure Ukraine to accept their losses in order to bring an end to the fighting”.

Zelensky told a Yale University summit of business leaders by video link on Wednesday that he believes Russia will not seek a diplomatic end to the war unless the world supports Ukrainian troops in their fight.

“We are an independent, righteous, normal country,” Zelensky said, adding about his troops’ war efforts: “We do it on our land and we slowly push them back. That’s how we’re going to keep on moving.”

Additional reporting by Associated Press and Agence France-Presse