Several shells slammed into a crowded UN school used as shelter for refugees in the Gaza war early on Wednesday, killing at least 15 people and wounding 90, a Palestinian health official and a UN official said.

The officials blamed Israel for the shelling. The IDF said it was looking into the incident.

The strike hit the UN school in the Jebaliya refugee camp in Gaza City, where hundreds of Palestinians had sought shelter from the intense fighting and Israeli tank fire, said Adnan Abu Hasna, a spokesman for the UN aid agency UNRWA, which has opened its schools to those displaced by the fighting.

Starting at around 4:30 a.m., several shells hit the school compound, a few minutes apart, said the school principal, Fayez Abu Dayeh. He said shells hit two classrooms and a bathroom.

In one of the classrooms, the front wall was blown out, leaving debris and bloodied clothing. Another strike tore a large round hole into the ceiling of a second floor class-room.

Emergency services workers at the school were carrying out the grim task of picking body parts off the blood-soaked floor of a bombed-out classroom, an AFP correspondent said.

The outer wall of the complex had also been damaged and a number of dead animals including donkeys could be seen lying on the ground.

Ashraf al-Qidra, a Palestinian health official, said at least 15 people were killed and about 90 wounded. A UN official put the death toll at 16.

The deadly strike came a day after Israel unleashed a heavy assault on targets in Gaza, destroying key symbols of Hamas control, shutting down the territory’s only power plant and leaving at least 128 Palestinians dead, according to Gazan officials.

Israel was blamed for a similar strike on a UN-run school last week that killed 16 people, but after an investigation, the IDF said while one shell had hit the school, it had not caused any casualties.

Palestinian health officials say some 1,260 Gazans have been killed in three weeks of fighting. Hundreds of them are Hamas fighters, according to Israel.

Some 215,000 Gazans have fled their homes, most taking shelter at UN-run facilities, according to the UN.