Ukrainian forces may have to retreat from their last pocket in the Luhansk region to avoid being captured, a Ukrainian official has said, as Russian troops press an advance in the east that has shifted the momentum of the three-month-old war.
A withdrawal could bring Russian President Vladimir Putin closer to his goal of capturing eastern Ukraine's Luhansk and Donetsk regions in full.
Russian troops have gained ground in the two areas collectively known as the Donbas while blasting some towns to wastelands.
Luhansk's governor, Serhiy Gaidai, said Russian troops had entered Sievierodonetsk, the largest Donbas city still held by Ukraine, after trying to trap Ukrainian forces there for days.
Mr Gaidai said 90 per cent of buildings in the town were damaged.
"The Russians will not be able to capture Luhansk region in the coming days as analysts have predicted," Mr Gaidai said on Telegram, referring to Sievierodonetsk and its twin city Lysychansk across the Siverskiy Donets River.
"We will have enough strength and resources to defend ourselves. However, it is possible that in order not to be surrounded we will have to retreat."
Russia-backed rebels said that they had taken over Lyman, Donetsk's large railway hub north of two more key cities still under Ukrainian control and west of Sievierodonetsk.
Ukrainian presidential adviser Oleksiy Arestovych acknowledged the loss on Thursday night, though a Ukrainian Defence Ministry spokesperson reported on Friday that its soldiers countered Russian attempts to completely push them out.
Ukraine said its forces were blocking a Russian advance to Sloviansk, to the south-west.
President Volodymyr Zelenskyy said Ukraine was protecting its land "as much as our current defence resources allow".
Ukraine's military said it had repelled eight attacks in Donetsk and Luhansk on Friday, destroying tanks and armoured vehicles.
"If the occupiers think that Lyman and Sievierodonetsk will be theirs, they are wrong. Donbas will be Ukrainian," Mr Zelenskyy said in an address.
'At great cost'
British Prime Minister Boris Johnson told Bloomberg UK that Mr Putin "at great cost to himself and to the Russian military, is continuing to chew through ground in Donbas".
Russian troops advanced after piercing Ukrainian lines last week in the city of Popasna, south of Sievierodonetsk.
Russian ground forces have captured several villages northwest of Popasna, Britain's defence ministry said.
Reached by Reuters journalists in Russian-held territory on Thursday, Popasna was in ruins.
The bloated body of a dead man in combat uniform could be seen lying in a courtyard.
Resident Natalia Kovalenko had left the cellar where she was sheltering in the wreckage of her flat, its windows and balcony blasted away.
She said a shell hit the courtyard, killing two people and wounding eight.
"We are tired of being so scared," she said.
Russia's eastern gains follow the withdrawal of its forces from approaches to the capital, Kyiv, and a Ukrainian counter-offensive that pushed its forces back from Ukraine's second city, Kharkiv.
Ukrainian forces have been unable to attack Russian supply lines to the Donbas.
Russian forces shelled parts of Kharkiv on Thursday for the first time in days.
Authorities said nine people were killed.
The Kremlin denies targeting civilians in what it calls its "special military operation" in Ukraine.
In the south, where Moscow has seized a swath of territory since the February 24 invasion, including the port of Mariupol, Ukrainian officials say Russia aims to impose permanent rule.
In the Kherson region, Russian forces were fortifying defences and shelling Ukraine-controlled areas, the region's Ukrainian governor, Hennadiy Laguta, told media.
He said the humanitarian situation was critical in some areas and people were finding it very difficult to leave.