The Kremlin on Wednesday accused Ukrainian forces of "provocative actions" as fresh clashes flared in east Ukraine after Kiev said pro-Moscow rebels had launched a new assault.

"In Moscow we are observing and feel acute concern over the provocative actions of the Ukrainian army, which, as far as we can judge, is to a large extent provoking the situation," Kremlin spokesman Dmitry Peskov said, state news wire TASS reported.

"We are deeply worried about the ongoing shelling of populated areas in Donbass with the deaths of adults and children, and therefore we are watching very closely how the situation develops and will formulate our position depending on developments," Peskov said.

Ukraine accused pro-Russian forces -- which it says are orchestrated by Moscow -- of launching a "large-scale offensive" against its positions close to the rebel bastion of Donetsk Wednesday morning in breach of a tenuous ceasefire deal.

Government troops had fought back against the attack using heavy artillery, it added in a statement.

Fighting has intensified around key hotspots on the frontline in eastern Ukraine despite a February peace deal, and the latest violence has revived fears that pro-Russian separatists may be readying for a new all-out attack against Kiev forces.

Senior rebel commander Eduard Basurin confirmed that fresh fighting had taken place around Maryinka, a settlement just to the west of Donetsk, but denied that rebel forces had taken control there.