Gaza City (CNN) -- At least 16 people were killed and many more were wounded when a U.N. shelter in northern Gaza was hit during Thursday's fighting, officials said.
John Ging, director of the U.N.'s Office for Coordination of Humanitarian Affairs, confirmed to CNN's Christiane Amanpour that 16 died in the strike. Numbers were still coming in as to how many people were wounded, he said.
A Palestinian government statement condemned the incident, calling it "Israeli brutal aggression that targeted" Gaza's displaced. It demanded an end to the "Israeli war machine."
The strike wounded more than 200, most of them women and children, the statement said. It also said the death toll was 16.
It's unclear who was behind the incident. The Israeli military said it could have been a rocket fired from Gaza that fell short of Israel and exploded.
U.N. Secretary-General Ban Ki-moon, who met with U.S. Secretary of State John Kerry in Cairo, was irate.
"I am telling to the parties -- both Israelis and Hamas, Palestinians, that it is morally wrong to kill your own people," he said. "Whole world has been watching, is watching with great concern. You must stop fighting. And enter into dialogue.
"Whatever grievances you may have, this is wrong. Why are you continuing to kill people? There are many other ways to resolve this issue without killing each other."
The coordinates of the school in Beit Hanoun, which was serving a shelter for families in Gaza, had been given to the Israeli military, said a U.N. spokesman, Chris Gunness.
Footage from the school showed pools of blood and images from hospitals showed absolute chaos. There were so many victims than many gurneys included two wounded children.
One father carried his small daughter into the hospital. There wasn't much the dad could do but try to comfort his little girl as she cried and begged for him not to leave her.
In another area a mortician wrapped up the body of a 1-year-old girl who was killed.
All the while people wandered through the halls, trying desperately to find where their loved ones had been taken.
A CNN crew that visited the school three hours after the hit discovered a one-inch deep hole in the concrete in the courtyard where people were killed and injured. It appeared shrapnel struck people within a 30-meter radius. Walls were hit as high as about eight meters above the ground.
CNN personnel didn't see the remnants of any rocket or missile.
Some witnesses told CNN there were three to four explosions.