"The fighting has reached its maximum intensity," Ukraine's Deputy Defence Minister Ganna Malyar told a press briefing.

"Enemy forces are storming the positions of our troops simultaneously in several directions. We have an extremely difficult and long stage of fighting ahead of us," she added.

Moscow's army has plotted a slow but steady course deeper into the Donbas region since withdrawing forces from central and northern regions to consolidate military efforts in the east.

They are closing in around several key urban hubs, particularly Severodonetsk and Lysychansk that stand on route to Ukraine's eastern administrative centre in Kramatorsk.

"The situation remains difficult and shows signs of further aggravation," Malyar said.

"We must understand that this is a war and, unfortunately, losses on our part are inevitable," she added.

The Lugansk region's governor Sergiy Gaiday said that "heavy" Russian bombardments on Lysychansk had done extensive damage to civilian infrastructure, including a humanitarian aid centre.

Gaiday said that three people had died in recent Russian attacks on the region, which Moscow's forces have said they have nearly complete control over.