A Ukrainian soldier stands guard on a road during a prisoners-of-war (POWs) exchange, north of Donetsk, eastern Ukraine on Sept. 28, 2014. Reuters/Marko Djurica

(Reuters) - Seven Ukrainian soldiers were killed when a separatist shell hit their armored personnel carrier near Donetsk airport in eastern Ukraine, a military spokesman said on Monday, putting further strain on a fragile ceasefire in the region.

It was the largest loss of life among Ukrainian soldiers in a single incident since the ceasefire began on Sept. 5.

"Seven (Ukrainians) were killed late last night not far from Donetsk airport," spokesman Andriy Lysenko told Reuters.

Separately, Yuri Biryukov, an adviser to President Petro Poroshenko, said on his Facebook page that the separatists had fired the shell from a tank. He said nine Ukrainian soldiers were wounded in the attack and that separatist tanks had been destroyed in return fire.

Despite the ceasefire, fighting has flared frequently around Donetsk international airport, which is held by government forces. The rebels hold key points inside the city itself, an industrial hub with a pre-war population of about one million.

Poroshenko called the ceasefire, under pressure from Western governments, after his troops suffered big battlefield losses against the separatists in August which Kiev ascribed to the direct intervention of Russian forces.

Moscow, which opposes the pro-Western course pursued by the former Soviet republic, denies its troops have played any direct part in the conflict or armed the separatists despite what Kiev and Western governments say is incontrovertible proof.

Last week, Poroshenko lauded the fact that military clashes had diminished and that military casualties had dwindled to zero, showing his peace plan was working.

A United Nations human rights official said on Sept. 23 that more than 3,500 people had been killed in the conflict, including 298 people who died when a Malaysian passenger plane flying over eastern Ukraine was brought down in July.