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, as well as setting buildings ablaze and leaving shell craters in roads.

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.