DONETSK, Ukraine — Heavy fighting broke out Sunday between Ukrainian and rebel forces on the north side of Donetsk, killing at least six people in one of the most serious clashes since the 10-day-old cease-fire came into force. The airport, where Ukrainian troops are almost surrounded, was on fire, billowing black smoke, and a house and a city market burned just off the main boulevard inside the city.

“It’s not a cease-fire. It’s full-on fighting,” a rebel fighter said. Tanks were out on the streets midafternoon, and pro-Russian rebel fighters raced reinforcements along the main boulevard in civilian cars toward their positions on the approaches to the airport.

In a residential area just off the main boulevard, hospital workers collected bodies from the sidewalks and homes hit hours earlier. They took at least six, most of them of older adults who appeared to have been walking home from the market, and a rebel policeman who had been trying to guide people to cover when he was hit.

Artillery fire resounded for hours, through Saturday night and much of the day Sunday, as Ukrainian forces at the airport and areas northwest of the city traded fire with the separatist forces that control the city and most of the surrounding territory. It was unclear who instigated the fighting or why, but the sound of rifle fire at the airport midafternoon indicated the two sides were engaged in close combat.