JEBALIYA REFUGEE CAMP, Gaza Strip (AP) — Israeli tank shells slammed into a crowded U.N. school sheltering Gazans displaced by fighting on Wednesday, killing 15 and wounding 90 after tearing through the walls of two classrooms, a spokesman for a U.N. aid agency and a health official said.

Israeli airstrikes and shelling killed more than 90 Palestinians across the coastal territory throughout the day Wednesday, including those killed at the school and multiple members of two families struck in their homes, health officials said.

The Israeli military said mortar shells had been fired from near the school, and that soldiers fired back.

Later Wednesday, the military declared a four-hour humanitarian cease-fire in parts of Gaza beginning at 3:00 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.

The fighting continued after the cease-fire was to have begun, with an Israeli airstrike killing seven Palestinians, according to Gaza health official Ashraf al-Kidra. The Israeli military said Palestinians fired several rockets after the start of the pause, and that its forces were still working to demolish tunnels in certain areas of the Strip.

The latest violence further dimmed hopes of a sustainable truce in the fighting, now in its fourth week. The strike at the U.N. school in the Jebaliya refugee camp came on the heels of Israel's heaviest air and artillery assault so far in the conflict.

Israel escalated its campaign on Tuesday, with airstrikes destroying key symbols of Hamas power, including the home of the top Hamas leader. Gaza's only power plant was shut down after shells set its fuel tank on fire.

On Wednesday, Israeli aircraft struck dozens of Gaza sites, including five mosques it said were being used by militants, while several other areas came under tank fire.

In Jebaliya, tank shells hit the U.N. school before dawn, said Adnan Abu Hasna, a spokesman for the U.N. Relief and Works Agency. The agency is sheltering more than 200,000 people displaced by the fighting at dozens of U.N. schools across the coastal strip.

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.

In one classroom, the front wall was blown out, leaving debris and bloodied clothing. Another strike tore a large round hole in the ceiling of a second-floor classroom. The wall of the lavatories was also damaged.

The Israeli military said it fired back after its soldiers were targeted by mortar rounds launched from the vicinity of the school.

The mortars were fired from a distance of some 200 meters (yards) from the school, said an Israeli military official, speaking on condition of anonymity because he was not authorized to talk to the media.

About two hours after the strike, hundreds of people still crowded the school courtyard, some dazed, others wailing.

Aishe Abu Darabeh, 56, sat on the ground with her relatives.

"Where will we go?" she asked. "Where will we go next? We fled and they (the Israelis) are following us."

Four of the dead were killed just outside the school compound, two in their home nearby and two in the street, after returning from pre-dawn prayers, their relatives said.

The bodies of two members of the al-Najar family, 56-year-old Shaher and his 41-year-old brother, Bassem, were laid out in one of the rooms of their small home, surrounded by wailing relatives. Outside the gate, another relative held on to his crying son, hugging him tight and saying: "I'm here, I'm not going anywhere."

Chris Gunness, another spokesman for the U.N. Palestinian refugee agency, said the international community must step in.

"We appealed incessantly to the political echelons with power to reduce and end the violence. We warned. Our voice was not heard. It is time for others to honor their international law responsibilities to deal with consequences on civilians of their military decisions," he said.

Al-Kidra, the Gaza health official, said at least 15 people were killed and about 90 wounded in the school strike.

In all, 92 Palestinians were killed by airstrikes and tank shelling in different areas of Gaza on Wednesday, al-Kidra said. The total number of Palestinians killed since the July 8 start of fighting rose to 1,331, he said. More than 7,100 Palestinians have been wounded.

Israel has lost 53 soldiers and three civilians.

In the southern town of Khan Younis, 10 members of one family died when an airstrike hit a relative's home where they had sought refuge, al-Kidra said.

After the strike, relatives climbed over piles of debris, surveying shattered windows and demolished walls.

"Me and my brother and one of my relatives, we were sleeping. And we tried to look through the window to see what happened. But we couldn't see anything because of the smoke. And when we came down, we saw everything was damaged," said Mohammed al-Astal, a relative.

In the Gaza City neighborhood of Tufah, shelling killed at last seven members of another family, including four children, said Ayman Sahabani, the head of the emergency room at Gaza's Shifa Hospital.

Gazans said they have not been able to properly mourn the mounting number of fatalities, with many foregoing funerals because of the security situation.

In Gaza City, three bodies wrapped in green tarp lay on the bed of a pickup truck, with a charred hand sticking up from one. Ordinarily, crowds of people would accompany bodies to cemeteries, but on Wednesday, only a small number of relatives rode among the bodies.

Israel says its Gaza operation is meant to stop Hamas rocket and mortar fire that has reached increasingly deeper into its territory and to destroy a sophisticated network of tunnels used for attacks inside Israel.

Gaza militants have fired more than 2,600 rockets toward Israel over the past three weeks, according to the Israeli army.

The Israeli military has said it is hitting targets linked to militants, such as rocket launching sites, weapon depots and Hamas tunnels. Over the past 23 days, Israeli forces have hit 4,100 targets in Gaza, about one-third connected to the militants' ability to launch rockets at Israel, a statement said.

The military has not provided details on strikes in which multiple members of one family were killed. There have been several dozen such strikes, according to the Palestinian human rights group Al-Mezan.

The military says Hamas militants often launch rockets from crowded residential areas, thus endangering civilians. The army says it has also warned civilians by phone and leaflet to leave dangerous areas.

On Wednesday, aircraft dropped leaflets over Gaza City's Rimal neighborhood, urging residents to stay away from Hamas militants and to report possible rocket launches. The leaflet gave a contact phone number and email and warned of a new operation.

Israeli leaders have said that troops will not leave until all the Hamas tunnels have been demolished. The army said 32 tunnels have so far been located but did not say how many remain. Since Tuesday morning, troops have demolished three more tunnels, a statement said.

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Enav reported from Jerusalem. Associated Press writers Ibrahim Barzak in Gaza City and Yousur Alhlou in Jerusalem contributed to this report.