(Reuters) -- Ukrainian troops launched a "mass operation" overnight retaking almost all the territory of Donetsk airport in eastern Ukraine lost to separatists in recent weeks, military representative Andriy Lysenko said Sunday. The army's offensive at the airport brought the fighting close to the city of Donetsk itself, center of a pro-Russian separatist rebellion. Residents reported intensified outgoing shelling, including from residential areas in central parts of the separatist-held city.

"The decision was taken for a mass operation ... We succeeded in almost completely cleaning the territory of the airport, which belongs to the territory of Ukrainian forces as marked by military separation lines," Lysenko said in a televised briefing. He said the operation had returned the battle lines near the airport to the previous status quo and that the Ukrainian army had thus not violated the Minsk 12-point peace plan agreed with Russia and separatist leaders last September.

Four Ukrainian soldiers were killed and 32 injured in the past 24 hours, Lysenko said.

With attempts to restart peace talks stalled, pro-Russian rebels have stepped up attacks in the past week, seizing from government troops parts of the airport, which is of strategic value to both sides.

"It was impossible to sleep -- explosions, the walls were shaking. It seemed like they were firing from near the building ... The DNR [rebel] army were firing from our district," advertising executive Alla, 53, said by telephone.

A cease-fire deal reached at the talks in Minsk, capital of Belarus, in early September was regularly violated from the start by both sides.

The World Health Organization says more than 4,800 people have been killed in the conflict, which pits Kiev's forces against separatists who the West says are supported and armed by Russia.

Despite what Kiev and the West say is incontrovertible proof, Russia denies its troops are involved or that it is funneling military equipment to the separatists.

With its runways pitted and cratered, the airport itself, with a multistory control tower and extensive outbuildings, has long since ceased to function. But its hulk, battered by shelling and gunfire, has taken on symbolic value for both sides, with troops and separatists hunting each other often at close range in a deadly cat-and-mouse game among the ruins.

(Reporting by Natalia Zinets and Lina Kushch; Writing by Alessandra Prentice; Editing by Richard Balmforth and Ralph Boulton)