"There was a major attack in the night" from Sunday to Monday in Kreminna, the Lugansk regional governor Sergiy Gaiday said in a statement on social media.

"The Russian army has already entered there, with a huge amount of military hardware ... Our defenders have retreated to new positions," he added.

Kreminna, with a pre-war population of nearly 20,000 people is around 50 kilometres (31 miles) northeast of Kramatorsk, the region's administrative centre, and is a strategic target for invading Russian forces.

Rubizhne, which is under control from Russian forces, was under intense fire from Ukrainian artillery and mortars, AFP journalists reported.

Powerful explosions could be seen in Rubizhne, sometimes followed by fires and plumes of white or black smoke.

Russian forces have stepped up their offensive to capture the eastern Donbas region of Ukraine having pulled back troops deployed around the capital Kyiv at the beginning of the invasion in late February.