Updated at 7:59 p.m. ET.

A United Nations-run school sheltering civilians in Gaza came under attack Thursday, the U.N. says. More than a dozen people have been killed, according to Palestinian officials.

Reuters quotes Chris Gunness, a spokesman for the U.N. Relief and Works Agency, the main U.N. agency in Gaza, as confirming that the shelter in Beit Hanoun was hit.

The Associated Press reports that "Gaza health official Ashraf al-Kidra says the dead and injured in the school compound were among hundreds of people seeking shelter from heavy fighting in the area."

Reuters says:

"Laila Al-Shinbari, a woman who was at school when it was shelled, told Reuters families had gathered in the courtyard expecting to be evacuated shortly in a Red Cross convoy." 'All of us sat in one place when suddenly four shells landed on our heads ... Bodies were on the ground, (there was) blood and screams. My son is dead and all my relatives are wounded including my other kids,' she wept."

Israel says it's looking into the incident. In a statement, the Israeli Defense Forces said militants opened fire at Israeli solders from the school area. The statement continues:

"In order to eliminate the threat posed to their lives, [the soldiers] responded with fire toward the origins of the shooting."

A Hamas spokesman accused Israel of "committing massacres."

U.N. Secretary-General Ban Ki-moon condemned the attack, calling on Israel and Hamas to abide by international humanitarian law and respect "the sanctity of civilian life.

"This is wrong," Ban said. "Why are you continuing to kill people?

"There are many other ways to resolve this issue without killing each other," Ban said. "I'm angry about what they are doing."

State Department spokeswoman Jen Psaki said the U.S. was "deeply saddened and concerned about the tragic incident" and "the rising civilian death toll in Gaza."

"This also underscores the need to end the violence and to achieve a sustainable ceasefire and enduring resolution to the crisis in Gaza as soon as possible," Psaki said.

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