"The city has been fully occupied by the Russians," mayor Oleksandr Striuk said.

Later on Saturday, the Russian defence ministry's spokesman Igor Konashenkov announced the "total liberation" of Severodonetsk as well as nearby villages Borivske, Voronove and Syrotyne.

"All territory on the left bank of the Donets river within the limits of the Lugansk region is under control" of Russian and pro-Russian forces, he said.

The Ukrainian army on Friday said it would withdraw its forces from the city of around 100,000 inhabitants before the war to better defend the neighbouring city of Lysychansk.

Both lie in the wider Lugansk region.

Striuk said civilians had started to evacuate the Azot chemical plant, where several hundred people had been hiding from Russian shelling.

"These people have spent almost three months of their lives in basements, shelters. That's tough emotionally and physically," he said, adding they would now need medical and psychological support.

Pro-Moscow separatists said Russian and pro-Russian forces had taken control of the Azot factory and "evacuated" more than 800 civilians sheltering there.

The forces have "taken full control of the Azot plant industrial zone", a separatist representative Andrei Marochko said on Telegram.

Another separatist spokesman, Ivan Filiponenko, said that around 800 civilians who had taken refuge in the plant during weeks of fighting had been "evacuated".

The Russians also taking control of Lysychansk would effectively give the them control of the wider Lugansk region, and allow them to push further into the wider Donbas.