IDF officials said Wednesday that Israeli soldiers fired several tank shells at a UN school used as a shelter for refugees in response to mortar shells fired by Gazan operatives from there. At least 15 people were killed in the IDF shelling, and 90 were wounded, according to Ashraf al-Qudra, a Palestinian health official.

The UN school, in the Jabaliya refugee camp in Gaza City, was housing hundreds of Palestinians who 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 Palestinians 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 classroom.

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.

Assad Sabah said he and his five children were huddling under desks in one of the classrooms because of the constant sound of tank fire throughout the night.

“We were scared to death,” he said. “After 4:30 a.m., tanks started firing more. Three explosions shook the school.”

“One classroom collapsed over the head of the people who were inside,” he said.

A Palestinian boy cries while receiving treatment for injuries caused by an Israeli strike at a UN school in Jebaliya refugee camp, at the Kamal Adwan hospital in Beit Lahiya, northern Gaza Strip, Wednesday, July 30, 2014 (photo credit: AP Photo/Khalil Hamra)

A Palestinian boy cries while receiving treatment for injuries caused by an Israeli strike at a UN school in Jebaliya refugee camp, at the Kamal Adwan hospital in Beit Lahiya, northern Gaza Strip, Wednesday, July 30, 2014 (photo credit: AP Photo/Khalil Hamra)

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 says its offensive in the Strip, launched in early July, is intended to counter rocket fire on its civilians and to destroy a vast network of attack tunnels used to target Israelis.

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 that it had only fired a single shell at the schoolyard and that the fatalities had been caused by a misfired missile launched by Islamic Jihad.

Later Wednesday, Israel declared a four-hour humanitarian lull in parts of Gaza beginning at 3 p.m. but Hamas spokesman Sami Abu Zuhri said it lacked any “value” because it excluded border areas from where Hamas wanted to evacuate the wounded.

Shortly after the lull went into effect, sirens warning of Hamas rocket fire sounded in several communities in southern Israel.

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. Fifty-three Israeli soldiers have been killed, as well as three civilians.

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