Ukrainian troops patrol near the city of Debaltseve in the Donetsk region of Ukraine on Aug. 3. (Anatolii Stepanov/AFP/Getty Images)

Pro-Russian separatists battled Sunday to keep advancing Ukrainian government forces at bay in heavy fighting on the outskirts of Donetsk, the rebels’ main stronghold in eastern Ukraine.

Shelling by Ukrainian troops, some of them in sunflower fields outside the large industrial city, killed six people over the weekend, city officials said.

The army has all but encircled the other main rebel redoubt of Luhansk, where three civilians were killed in the latest fighting, and is trying to tighten the noose around Donetsk.

Fighting has intensified since the downing of a Malaysian airliner in rebel-held territory on July 17. Meanwhile, with each side blaming the other for the deaths of the 298 crew members and passengers, relations between Russia and the West are deteriorating rapidly.

British Foreign Secretary Philip Hammond appealed to Russian President Vladimir Putin to persuade the rebels to end the conflict and to ensure they do not hinder international experts trying to recover human remains from the plane’s wreckage.

“It is an unspeakable abomination that two weeks after this crash there are still bodies on the crash site unrecovered and the Russians have not used their influence with the separatists,” Hammond told the Sunday Telegraph newspaper.

“I said last week that there is one man who can snap his fingers and make this happen, and he hasn’t done so. He must now do so,” Hammond said, urging Putin to cut off arms to the rebels and “stop destabilizing and interfering” in Ukraine.

Russia has been accused of arming the rebels and trying to orchestrate events in Ukraine since the February ouster of a president sympathetic to Moscow. Putin denies the allegations and accuses the West of attempting to “contain” Russia.

A military spokesman in the Ukrainian capital, Kiev, said the army had recaptured three-quarters of the territory the rebels once controlled in their self-proclaimed “people’s republics” of Luhansk and Donetsk.

In central Donetsk, shelling echoed for hours and smoke from burning buildings in the outlying Petrovsky district stretched as far as the city center.

In another outlying district, Maryinka, roads and houses were pockmarked by shells and some buildings were burnt-out carcasses.

Many homes in Luhansk, which is close to the border with Russia, also have no electricity or water.

The Ukrainian military said it had suffered no losses in the latest fighting.

Efforts to end the conflict, in which the United Nations says more than 1,100 people have been killed, have stalled.

The United States and the European Union have imposed sanctions on Russia over its role in Ukraine but say Putin has done nothing to show that he is changing policy.

Dutch and Australian forensic experts have gained access to the site of the downed Malaysia Airlines Flight 17 for three successive days and have recovered more human remains to send back to the Netherlands for forensic tests. Victims included 196 Dutch, 27 Australians and 43 Malaysians.

The United States suspects the separatists erroneously shot down the Boeing 777 with a Russian-supplied missile, but Moscow denies that and blames the disaster on Kiev.