Pro-Russian rebels in eastern Ukraine called on Tuesday for an emergency UN Security Council session that could authorise sending international peacekeepers into the war zone.

The leadership of the self-proclaimed Donetsk People's Republic said in a statement it was seeking "the intervention of a peacekeeping force that includes Russian representatives to control the implementation of existing agreements and help resolve the humanitarian and social situation".

The Ukrainian government signed a ceasefire with the separatist rebels in September but it has failed to end the seven months of fighting.

Almost 1,000 people have been killed since the truce, the United Nations said last week -- an average of 13 people a day.

The Donetsk authorities said on Tuesday that three more civilians had been killed by shelling in the past 24 hours, including two killed when a shell hit a minibus near the city airport that has been the scene of some of the fiercest battles.

There are reports that more troops and weaponry have come across the border with Russia in the past week, raising fears that a fresh offensive against government forces may be imminent.

There are currently between 15,000 and 20,000 armed rebels in the conflict zone, a Ukrainian military spokesman claimed on Tuesday, as well as "5,000 to 10,000 Russian professional soldiers who are present on Ukrainian territory".

Russia denies directly supporting the rebels and has claimed that any Russian troops in the conflict zone are volunteers.

The ceasefire has become ever more fragile since rebels defied the government and held elections in early November that they described as legitimising their two self-declared independent states.