Russian forces have now taken control of the devastated city of Severodonetsk in eastern Ukraine, its mayor said.

"The Russians have completely occupied Severodonetsk, our army has withdrawn to more prepared positions," said Alexander Stryuk, reports Telegrafi.

This city was mostly reduced to rubble after weeks of heavy Russian shelling.

Many civilians have been sheltered at the large Azot chemical plant, but Stryuk said the only way out now was through Russian-controlled territory.

Pro-Russian rebels, allied with the Russian military, say they have moved to parts of Lysychansk, the town opposite Severodonetsk across the Siversky Donets River.

But Ukrainian officials have not confirmed this.

Severodonetsk, once home to about 100,000 people, has seen fierce street fighting and most of its residents have fled.

The capture of the city means that Russia now controls almost the entire Luhansk region and most of neighboring Donetsk - the two regions that make up the industrial Donbas.