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Evgeniy Maloletka, Associated Press
Ukrainian troops ride on tanks near Artemivsk, eastern Ukraine, Tuesday, Feb. 24, 2015. Ukrainian officials said they haven’t yet started pulling heavy weapons back from a frontline in eastern Ukraine because of continued rebel violations of a cease-fire deal.

DONETSK, Ukraine — Ukrainian troops held captive in the separatist stronghold of Donetsk began digging through the rubble Wednesday to retrieve the bodies of fellow soldiers killed last month in a bitter battle for the city's airport.

Associated Press journalists saw at least four bodies being carried out of the waste of the once-glittering, now-obliterated terminal. Rebel representatives said many more were still buried under the rubble, but provided no figures.

Russian-backed separatists and Ukrainian troops had battled regularly over Donetsk's airport since May, when government forces decisively rebuffed separatist attempts to take the showcase terminal built to help Ukraine host the 2012 Euro soccer championships. Fighting there then surged in mid-January, swiftly unraveling a monthlong truce over the New Year.

Ukrainian Defense Ministry spokesman Vladislav Seleznev said Wednesday that 15 servicemen were killed in fighting over the airport in January. Their bodies have lain uncollected since that time.

Buckling under a barrage of artillery and small arms attacks, Ukrainian forces conceded Jan. 22 that they had lost much of the terminal.

The rebels handed over 139 captive Ukrainian soldiers last weekend in exchange for 52 people held by the government. Neither side has revealed how many captives they currently hold, but AP journalists saw up to 25 government POWs working at the airport.

Separatists have long insisted they are entitled to control over the international airport, now reduced to mounds of smashed concrete. Ukraine argued that a peace deal signed in September entitled them to keep the terminal.

Work to retrieve the bodies was interrupted Wednesday by the sounds of gunfire in the distance, but resumed after a brief period.

The fighting in eastern Ukraine has killed nearly 5,800 people since April. Russia denies charges that it is arming and supporting the rebels, but Western nations and NATO reject those denials as absurd. A peace plan agreed upon earlier this month by the leaders of Russia and Ukraine, brokered by France and Germany, aims to cement a cease-fire and begin a pullback of heavy weapons.

Ukraine's military said rebel violations of the cease-fire persisted Wednesday but had fallen off in recent days.

Military spokesman Lt. Col. Anatoliy Stelmakh said the rebels shelled the village of Popasna twice overnight and also continued trying to overrun Ukrainian positions at the village of Shyrokyne, near the strategic port city of Mariupol. In all, 19 artillery and mine attacks were recorded, he said.

"For a second day, we observe a reduction in shelling," Stelmakh said. "But the last day was not a cease-fire."

Ukraine says it will not begin a pullback of heavy weapons under the peace deal until the latest cease-fire firmly takes hold. Rebels said Tuesday they were beginning a pullback, but the claim could not be independently confirmed.

The cease-fire deal is being monitored by the Organization for Security and Cooperation in Europe. OSCE mission chief Ertugrul Apakan said Tuesday that the warring sides had yet to provide figures on the quantity of heavy weapons to pulled back from the front line and where they would be moved.

"The parties concerned still have not provided this information, and they need to do so promptly. Without this information, the SMM cannot effectively verify the withdrawal," he said.

Jim Heintz and Peter Leonard in Kiev, Ukraine, contributed to this report.