Palestinian Abdel Aziz Jendiyah, 4, enters the family house, partially destroyed by an Israeli strike earlier during the war, in Gaza City. The normal entrance to the house was destroyed and now is filled with rubble, so the family had to open the hole on the wall for entry and exit.

JERUSALEM — Israel said today it has called up another 16,000 reservists, allowing it to potentially widen its Gaza offensive against the territory’s Hamas rulers in a three-week-old war that has killed more than 1,300 Palestinians and more than 50 Israelis.

The new call-up follows another day of intense fighting, in which tank shells struck a U.N. school where Palestinians were sheltering and an airstrike tore through a crowded Gaza shopping area. At least 116 Palestinians and three Israeli soldiers were killed Wednesday alone.

An Israeli defense official said the purpose of the latest call-up was to provide relief for troops currently on the Gaza firing line. However, Israeli officials have also said they do not rule out broadening operations in the coming days.

The move coincides with stalled diplomatic efforts to end the war, which has claimed more than 1,360 Palestinian lives — most of them civilians — and reduced entire Gaza neighborhoods to rubble since it began on July 8.

Israeli attacks in the strip continued today, with witnesses saying munitions struck the Omar Ibn al-Khatab mosque next to a U.N. school in the northern town of Beit Lahiya. Israeli fire near a U.N. school killed at least 17 people the day before.

Fifty-six Israeli soldiers and three civilians on the Israeli side have died in the Gaza campaign, as Palestinians have fired hundreds of rockets at Israel — some reaching major cities — and carried out attacks through tunnels beneath the heavily guarded frontier.

Israel has now called up a total of 86,000 reserves during the Gaza conflict, which it launched to try to end the rocket fire from Hamas and other militant groups.

An initial aerial campaign was widened into a ground offensive on July 17. Since then the campaign has concentrated on destroying more than 30 cross-border tunnels militants have constructed to carry out attacks on Israeli territory.

Israel says most of the 32 tunnels it has uncovered have now been demolished and getting rid of the remainder will take no more than a few days.

The strike in Beit Lahiya early today damaged water tanks on the roof of a building near the mosque, sending shrapnel flying into the adjacent school compound, where dozens of Palestinians displaced by the fighting had taken shelter.

“The shrapnel from the strike on the mosque hit people who were in the street and at the entrance of the school,” said Sami Salebi, an area resident.

Gaza health official Ashraf al-Kidra said at least 15 people were wounded, with three of them in critical condition.

Kifah Rafati, 40, was being treated for shrapnel injuries at the nearby Kamal Adwan hospital.

She said she and her six children had been sleeping in a classroom facing the mosque when the explosion went off. “There is no safety anywhere,” she said.