Seven Ukrainian soldiers have been killed in intensified clashes with pro-Russian rebels in the eastern conflict zone in the past 24 hours, the Kiev military said on Monday, reporting fierce fighting at the strategic small town of Debaltseve.

Violence in eastern Ukraine is at its worst level since a ceasefire was agreed last September and casualties have mounted, including in the port city of Mariupol where Kiev says 30 civilians were killed in rebel shelling on Saturday.

After months of skirmishes on the frontline, rebels say they were left with no choice but to launch an advance to push back government forces and prevent them from shelling cities under rebel control.

The Kiev government sees the rebel advance as a repudiation of the five-month-old ceasefire, restarting an all-out war in which 5,000 people have been killed. NATO says the advance is being carried out with the assistance of Russian troops on the ground, which Moscow denies.

"Rebels are constantly attacking Ukrainian government positions across the conflict zone with artillery, mortars, grenade launchers, tanks," spokesman Volodymyr Polyovy said at a televised briefing.

A further 24 soldiers were wounded in the past day, he said.

The pro-Russian rebels have vowed to encircle Debaltseve, a town with a population of around 26,000 that straddles the main road and train line between the two principle rebel strongholds, Donetsk and Luhansk.

Government troops are holding their positions in the town in the face of intensified attack, Polyovy said.

The military has reported civilian casualties at Debaltseve without giving any figures. Shelling has cut off the town's electricity and gas supplies and knocked out phone lines.

Polyovy also said Russia, which Ukraine and the West accuse of supporting the separatists, was beefing up its air force in Crimea, the peninsula annexed by Moscow from Kiev last March.

"Russia is increasing the fighting capacity of its air force on the territory of occupied Crimea," he said.