Israeli military fire hit a United Nations-run school in Gaza today, killing at least 20 people and injuring an estimated 90 people. The school under attack, called the Abu Hussein girls’ elementary school, is located in the densely-populated Jabaliya refugee camp.

The United Nations Relief Works and Agency (UNRWA), the group that serves Palestinian refugees, issued a stern statement placing the blame for the attack on the Israeli army.

“Last night, children were killed as they slept next to their parents on the floor of a classroom in a UN designated shelter in Gaza. Children killed in their sleep; this is an affront to all of us, a source of universal shame. Today the world stands disgraced,” said UNRWA Secretary General Pierre Krähenbühl. “We have visited the site and gathered evidence. We have analysed fragments, examined craters and other damage. Our initial assessment is that it was Israeli artillery that hit our school, in which 3,300 people had sought refuge.”

Krähenbühl added that the Israeli attack violated international law, and that UNRWA had informed Israel of the location of the school 17 times. The Palestinians who were sheltering there had been told by the Israeli military to flee their Gaza neighborhoods, only to be hit by Israeli shells at the place they thought would be safe. An estimated 240,000 displaced Palestinians are being sheltered in UNRWA facilities.

Israeli army spokespeople claim that Palestinian fighters fired from near the school–a claim they have frequently made when confronted by their attacks on civilians.

It was the second time in two weeks that an attack on a UN school caused deaths. Israel denies it hit the UN school in Beit Hanoun it bombarded last week, though Gaza-based journalists like The Daily Beast’s Jesse Rosenfeld have cast doubt on those claims, writing that the evidence appears to indicate Israeli fire hitting the area. The attack killed 16 people. UNRWA spokesman Chris Gunness said last week that three other UNRWA installations had been fired on by Israel, injuring five Palestinians in one incident.

UNRWA has also had to contend with Palestinian rockets being stored in schools that were abandoned. But there is no evidence that the schools hit by Israeli fire have rockets in them.

The attack on the UNRWA school in Jabaliya came after another night of heavy Israeli bombardment. The Gaza Ministry of Health said that over 70 Palestinians were killed since midnight. That brings the death toll to well over 1,200 Palestinians, the vast majority of them civilians.

Israel declared what it called a “humanitarian window” for four hours today–but also said it would continue to operate militarily in areas where soldiers were already firing. Hamas called it a declaration meant for the media and did not halt its fire. Israeli shells killed at least five Palestinians during the “humanitarian window, Ma’an News Agency reported. And another Israeli attack on a market in the Gaza neighborhood of Shuja’iyeh killed at least 15 people.