Ukrainian forces were on Sunday resisting a Russian assault on Sievierodonetsk, the largest city they still hold in the eastern Donbas region, but were weathering heavy artillery barrages, Ukrainian officials said.

The shelling was so intense it was not possible to assess casualties and damage, the governor of Luhansk region, Serhiy Gaidai, said. Dozens of buildings have been destroyed in the past few days.

"The situation has extremely escalated," Gaidai said.