Pro-Russian fighters launched an assault to control the city of Mariupol in Ukraine.

Rebels have announced an offensive and launched an assault on Saturday on Mariupol, a major port of 500,000 people, where Kiev said thirty civilians were killed by shelling.

Kiev officials said the offensive continued on Sunday along other areas in the front, which winds through two eastern provinces partially controlled by the separatists.

"Rebels are attacking the positions of anti-terrorist operation troops extremely intensively, using artillery, mortars, grenade launchers, tanks," military spokesman Andriy Lysenko said in a televised briefing.

He said four Ukrainian servicemen had been killed and 17 injured in the past 24 hours and reported that rebel attacks on the town of Debaltseve, northeast of separatist-held Donetsk, had been particularly fierce.

Rebel leader Alexander Zakharchenko said on Saturday the separatists planned to encircle Debaltseve, which has a population of around 26,000.

NATO said it has indications that a major rebel offensive is coming, due to heavy weapons moving in from Russia as they have prior to previous rebel pushes.

Separatist leader Alexander Zakharchenko said rebel fighters would continue to fight for more territory, and were advancing in five directions to push back Ukrainian government forces to the limits of the eastern Donetsk region.