Ukraine says its forces killed 12 rebels at Donetsk airport

(Reuters) - Pro-Russian separatists have suffered their worst casualties since a ceasefire officially began on Sept.5, losing 12 men in attacks on buildings at Donetsk airport, Ukrainian military officials said on Saturday.

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 in the rebel stronghold of Donetsk.

"The airport of Donetsk remains the priority target for terrorists. Yesterday they resorted to a few, fortunately unsuccessful, attempts to storm it," military spokesman Andriy Lysenko told journalists.

"Twelve (separatists) were killed during the attacks and that is the biggest single loss among rebels since Sept.5," he added.

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

Ukrainian officials accused Russian forces on Friday of helping separatists to step up pressure on government troops holding the airport in Donetsk, threatening a fragile ceasefire.

The latest U.N. 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.