Kreminna, a city of more than 18,000 people about 100 kilometres southeast of the capital Kyiv, appears to be the first city captured in a new Russian offensive in eastern Ukraine.
"Kreminna is under the control of the 'Orcs' (Russians). They have entered the city," Serhiy Gaidai, the governor of the Luhansk region, told a briefing.
"Our defenders had to withdraw. They have entrenched themselves in new positions and continue to fight the Russian army."
He said Russian forces had attacked "from all sides".
"It is impossible to calculate the number of dead among the civilian population," he said.
"We have official statistics - about 200 dead - but in reality there are many more, "
Gaidai did not make clear what period the estimated death toll covered.
Russia, which launched what it calls a "special military operation" on February 24, denies targeting civilians.
Ukraine said on Monday that Russian forces, after regrouping, had launched a new offensive focused on the east of the country.
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