Crackling forth over radio, the voice ordered tanks to take position in a residential area in Donetsk, the stronghold of separatist rebels in east Ukraine.

The rebel armored vehicles trundled to their destination accompanied by a reinforced minivan carrying infantrymen to provide cover. Minutes later, the shells blasted toward government positions. Out of ammunition, the tanks left the scene. Later the same morning, artillery flew in the opposite direction — this time seemingly from Ukrainian troops, hitting a severely damaged area still inhabited by hundreds of families.

Artillery duels like this one have been a dominant feature of Ukraine's conflict, which the United Nations estimates has claimed at least 6,454 lives among both combatants and civilians. After months of relative tranquility ushered in by an internationally brokered armistice in February, civilian deaths are again becoming commonplace amid the stepped-up heavy weapons fighting.

UNICEF, the United Nations children's agency, said that at least 68 children have been killed in the conflict — many by artillery. UNICEF said some children were injured by heavy weaponry as recently as last week, when heavy fighting erupted in Donetsk suburb of Marinka.

The rebel artillery last week hit a checkpoint, killing a rebel fighter and wounding at least two others. More often than not, the shells are less precise and slam into homes, shops or a public building, killing or wounding civilians.

Rebels say they are fighting to push back Ukrainian forces from their main city and keep them as far away from the civilian population as possible. Ukrainian officials say separatists are looking to provoke a new round of full-blown war.

What AP reporters have been able to observe looks like hit-and-run strikes by rebel forces, often carried out from inside residential zones. Some Donetsk inhabitants are willing to confirm as much.

"There is outgoing fire and then, after a while there is incoming shelling. This has been going on for a year," said Valentina Barkova, as she cleared away debris from a strike on an apartment block neighboring her own over the weekend.

Tensions are at a breaking point for residents. On Monday, a spontaneous rally broke out demanding an end to the war. Some locals are calling for a big push to thwart the Ukrainian army, but many allied to the insurgents are also calling on them to pull their weapons out of residential areas.

Rebel commanders insist that they have no heavy weapons on the front, despite evidence to the contrary.

"There are no such weapons here. We pulled them back. We use small arms and that's that. We are not violating the (peace) agreements even in the slightest way," said the head of the rebel unit at a front line position near the remains of Donetsk airport. The commander identified himself only by the nom de guerre Petrovich.

Separatist spokesman Eduard Basurin said Friday that two civilians had been killed as a result of Ukrainian shelling the previous day.

"During the violation of the cease-fire regime by Ukrainian forces, there were 11 casualties, among them two men killed in (Donetsk's) Kuibyshev district," he said.

One of the men killed Thursday was sitting outside drinking when the shell struck, his friend told The Associated Press. The shell tore into a dusty side-road by a rusting kiosk, and the shockwaves blew the man off his feet into a nearby fence.

Only meters (yards) away, the recently left tracks of heavy armor were still visible, although the vehicle had long gone. In the intense early summer sun, track-marks often remain impressed in softened asphalt as evidence the tanks have been there.

Ukraine's army has always denied it aims artillery at residential areas. The sheer scale of destruction across areas of Donetsk and other rebel towns and cities have strained that official line beyond breaking point.

On-site investigations by observers from the Organization for Security and Cooperation in Europe, which is ostensibly charged with monitoring the cease-fire, have also suggested shells have been aimed from government positions.

Last week, an AP reporter traveled in the afternoon to a Ukrainian government artillery position in an open field several kilometers from Pisky, a village just west of Donetsk.

Pisky has long been a headache for the separatist fighters and residents of Donetsk alike, because it has been a launching pad for much Ukrainian shelling. Accordingly, it has been on the receiving end of much artillery lobbed by rebel forces.

Around an hour-and-a-half after sunset, from the fields outside Pisky, AP observed what appeared to be incoming fire from rebel areas. Within an hour, a Ukrainian artillery position in the distance was observed undertaking what appeared to be a return salvo.

Heavy artillery has tended to be so imprecise that its use, particularly when aimed at residential areas, has come under critical scrutiny. Its only immediately obvious effect is to as act a deterrent — terrifying the opponent into desisting from taking position and initiating any fresh attacks.

The shells that hit the rebel-held city of Horlivka on Thursday left at least three dead, according to separatist officials.

"The shelling started all of a sudden, so we began to escape. When we ran outside, we saw a house burning," said Dmitry Nosov, holding his son Misha in his arms. "Here is a child that suffers from all this. How is it that this is our fault?"

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Associated Press reporter Evgeniy Maloletka contributed to this report from near Donetsk, Ukraine.