Several Israeli tank shells landed Tuesday at a playground in Gaza City, killing three children and their grandfather and injuring 12 other children and women, hospital sources and witnesses said.

Eyewitnesses said that seven tank shells slammed a playground where children were playing soccer, adding two other shells crashed through the ceiling of a nearby house, injuring six women.

Relatives of those killed said they prevented a group of Palestinian militants from firing mortars into Israel from an area that is adjacent to their houses just half an hour before Israeli tanks fired the shells.

But militants waited until people went for prayers at the neighborhood’s mosque and sent a round of mortar shells beyond the Israel-Gaza borderline, which is a little less than half a mile away from the bombed area.

“I was going out of the mosque when the shells hit the kids,” said Mohammed Helo 42, the children's uncle, at the morgue of Shifa hospital in Gaza, where the bodies were taken. “I did not know what was going on. All I heard was thunderous explosions then the moans of people who were just walking by. Limbless bodies were scattered all around.”

Adham Abu Selmiya, head of the emergency department in the Hamas Ministry of Health, said three children and a man from the same family were killed in the attack. He said the children ranged in age from 9 years old to 13 years old.

Hamas rulers of Gaza said what happened was a war crime, stressing that Israeli attacks will not keep the Palestinian people to from defending themselves.

“We are doing our best to avoid violence with Israel, but the Israelis continue to escalate their aggressions against us,” Hamas spokesperson Sami Abu Zuhri told reporters in Gaza.

The Israeli army confirmed the shelling and apologized for the killing of civilians, ordering an investigation into the incident.

The new escalation at the Gaza-Israel borders threaten an informal cease-fire that Hamas kept since the 22-day Israeli offensive on Gaza ended early 2009.

Hamas and other Gaza militants have fired at least 60 homemade missiles and mortar shells at Israel since Saturday.