Fresh fighting in the rebel stronghold of Donetsk has killed at least 12 rebels, as pro-Russian separatists suffered their worst losses since a ceasefire with Ukraine officially began on September 5.

Ukrainian forces clashed with rebels for control of the strategic Donetsk airport, in the restive east on Saturday, as blasts echoed across the rebel-held city.

"The airport of Donetsk remains the priority target for terrorists... twelve [separatists] were killed during the attacks and that is the biggest single loss among rebels since September 5," military spokesman Andriy Lysenko told the Reuters news agency.

Lysenko said that two Ukrainian servicemen were killed during the past 24 hours, but he gave no further details.

The status of the transport hub remained in dispute, with both separatist fighters and Kiev claiming control.

Separatist "prime minister" Alexander Zakharchenko told Russia's RIA Novosti news agency that his men controlled 95 percent of the sprawling structure.

On Friday, Ukrainian officials accused Russian forces of helping the rebels, threatening a fragile ceasefire.

The weeks-long battle for the prized but no longer operating airport, has been one of many violations of a Russia-backed ceasefire signed on September 5.

Kiev and the rebels, who agreed on September 19 to withdraw heavy weapons from a 30km buffer zone along the eastern front line, have blamed each other for violating the deal.

The ceasefire in eastern Ukraine has become increasingly frayed in recent days, leading to the death of a number of civilians and soldiers as well as a Red Cross worker.

The latest UN estimate is that more than 3,500 people have died in the conflict which erupted after pro-Western leaders took power in Kiev following street protests that chased Moscow-backed president Viktor Yanukovich from power.