DONETSK, Ukraine -- Ukrainian POWs in the separatist stronghold of Donetsk began a task Wednesday that strained their hearts as well as their muscles: digging through the rubble to retrieve the bodies of fellow soldiers killed last month in the bitter battle for the city's airport.

Associated Press journalists saw at least four bodies being carried out of the once-glittering, now-obliterated Donetsk airport terminal. A Ukrainian official said seven in all were retrieved. Rebel representatives said many more soldiers were still buried under the collapsed building, but provided no figures.

One captive soldier saw two friends being pulled out of the rubble, as the facility's twisted steel beams and smashed cement walls were being sawn into pieces and towed away.

"I recognized them from their clothing. They were my friends," said the man, a member of the Ukrainian army's 90th brigade who identified himself only as Sasha.

The bodies themselves were contorted by rigour mortis after being left outside for weeks in the frigid winter. Work was briefly interrupted by the sounds of gunfire in the distance, then resumed.

It was not clear whether the Ukrainian soldiers were forced into performing the recovery work or volunteered, but rebels have previously forced POWs to perform hard labour. The Ukrainian captives were assisted by rescue workers employed by the separatists.

"These guys were fighting here. I don't know what for. They were following the orders of their president, and they respected that order," said rebel commander Mikhail Tolstykh, known widely by nom de guerre Givi. "We all are military men here and we have to respect our enemy."

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 over the terminal surged in mid-January, swiftly unraveling a monthlong truce.

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

Ukrainian Defence Ministry spokesman Vladislav Seleznev has said 15 servicemen were killed in fighting over the airport in January. Their bodies have lain uncollected since then.

Vasily Budik, an adviser to Ukraine's Defence Minister, wrote on his Facebook page that seven bodies were recovered Wednesday and that work at the airport would continue.

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

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."

An AP journalist in Shyrokyne said it was mostly quiet Wednesday afternoon except for brief tank fire at government positions.

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 by monitors from the Organization for Security and Cooperation in Europe.

On top of the fighting in the east, Ukraine is also trying to contain severe economic troubles exacerbated by corruption and the cost of the war.

The currency, the hryvnia, has fallen about 70 per cent over the past year and dropped about 9 per cent on Monday. National bank head Natalia Gontareva said Wednesday the bank had bought about $80 million in foreign currency to stabilize the hryvnia.

Piling on the economic pressure, Russian President Vladimir warned Wednesday that Russia would cut gas supplies to Ukraine unless it paid in advance for future deliveries. He said the latest payment from Ukraine would only be good for another three to four days of gas, and warned that any Russian gas cutoff to Ukraine may disrupt supplies heading to other European nations.

In Brussels, European Union President Donald Tusk warned that the European Union will not hesitate to impose new punishment on the separatists and Russia if the latest Ukraine peace deal collapses.

Tusk told the EU legislature that "additional sanctions remain on the table. We should be ready for any development -- good or bad."