At least 15 people were killed, the Gaza Health Ministry said, after artillery hit a school which had been converted into a United Nations shelter.

JABALIA, Gaza Strip—The United Nations blamed Israel for the second deadly assault in a week on a shelter for displaced Gazans and warned of a dire humanitarian crisis with many in the Palestinian territory already lacking electricity, potable water or safe refuge.

The strike on the U.N.-run shelter early Wednesday killed 15 people, according to Gaza health ministry. Hours later, Israel declared a brief, unilateral pause in the bombardment. As Palestinians used the four-hour window to stock up on provisions, explosions rocked a crowded Gaza City market, killing 17.

Related Video: As the conflict between Israel and Palestinians in Gaza escalates, many Israelis are turning to technology. The popular Red Alert: Israel app provides users with up-to-the-minute information on rocket attacks from Gaza.

The carnage highlighted the desperate scramble in blockaded, densely populated Gaza to stay clear of the 3-week-old conflict between Israel and Hamas, the Islamist group that governs the territory. More than 1,350 Palestinians have been killed, according to the health ministry. Palestinian and U.N officials said most were civilians.

The U.N. Relief and Works Agency, or Unrwa, said more than 200,000 of Gaza’s 1.8 million residents are living in its 85 shelters. Many came after leaving home under Israeli warnings to evacuate combat zones.

Fifty-six Israeli soldiers have also been killed, including three who died Wednesday while uncovering a booby-trapped tunnel shaft inside a residence in southern Gaza, the military said. Three civilians have been killed by rockets fired into Israel.

At least 15 people were killed in shelling on a United Nations-run school sheltering Palestinians in the Gaza Strip on Wednesday.
Mohammed Salem/Reuters

The fighting this week has pushed living conditions in Gaza to a crisis point, according to the U.N. and international relief organizations. Israeli fire destroyed Gaza’s only power plant on Tuesday, leaving much of the territory without electricity. The outage has disabled sewage pumps, causing waste to run in city streets. The fighting has also disrupted water treatment systems.

“Our shelters are overflowing,” said

Pierre Krahenbuhl,

Unrwa’s commissioner-general. “Tens of thousands may soon be stranded in the streets of Gaza without food, water and shelter if attacks on these areas continue.”

About 3,300 people were crowded into the shelter at the Jabalia Elementary Girls School in northern Gaza when three blasts tore through the building just before dawn, leaving floors and mattresses in classrooms smeared with blood and strewed with children’s shoes.

“Children were killed as they slept next to their parents,” said Mr. Krahenbuhl. A guard at the school was killed.

Mr. Krahenbuhl condemned what he called a “serious violation of international law by Israeli forces.”

Abdul Kader Al Khatib

sat in the school courtyard nursing a shrapnel wound in his shoulder, barely able to speak. He said he had arrived at the shelter from Beit Lahiya, also in northern Gaza, after Israel’s military warned him to leave his home, which was subsequently destroyed by bombardment.

“Now they have bombed me here,” he said.



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Palestinian emergency personnel and civilians move the victim of an Israeli air strike on a market to an ambulance in the Shejaiya neighborhood Wednesday.
Agence France-Presse/Getty Images

Unrwa’s top official in Gaza,

Robert Turner,

said a U.N. team’s study of shrapnel and craters at the school concluded it had been hit by Israeli artillery, fired from the direction of Israel.

An Israeli military spokeswoman said Hamas militants had fired mortars at soldiers in the area and “the soldiers responded by firing toward the origins of the fire.” She couldn’t confirm whether the soldiers had fired artillery, adding the strike was under investigation.

The Obama administration condemned the shelling but stopped short of blaming Israel.

Marie Harf,

a State Department spokeswoman, said Israel should take additional steps to avoid civilian casualties and that U.N. facilities shouldn’t be targets.

The White House said it was extremely concerned that thousands of displaced Palestinians, who had been warned by the Israeli military to leave their homes, weren’t safe in U.N. shelters in Gaza.

Israeli officials countered with finger-pointing at Unrwa. The military has found rockets stored at three U.N. shelters during the offensive and U.N. officials have denounced militant groups for placing them there.

“Unrwa, instead of condemning Israel should make a real effort to stop being manipulated and used by the terrorist organizations,” Deputy Foreign Minister

Tzachi Hanegbi

said.

Israeli forces have fired on six Unrwa shelters this month, U.N. officials said. 16 people were killed on July 24 in the shelling of a school in Beit Hanoun that was sheltering hundreds, hospital officials said.

The military acknowledged firing a single errant mortar into the yard of that school, but said the yard was empty at the time and rejected claims that people were killed on the premises as a result. U.N. officials said the school was hit several times.

Israeli officials say their campaign, which began on July 8, aims to degrade Hamas’s rocket arsenals and network of cross-border tunnels used by militants to infiltrate Israel and carry out attacks.

Without power to charge their cellphones or watch TV, many Palestinians got only limited information about Israel’s four-hour humanitarian pause in the fighting on Wednesday, which a Hamas spokesman dismissed as a “media stunt.”

A Palestinian woman carries her belongings at a U.N.-run school sheltering civilians displaced by an Israeli ground offensive that was hit by shelling Wednesday in the city of Jabalia.
Reuters

Guy Inbar, a Defense Ministry spokesman, said Arabic-speaking Israeli officials gave two interviews—one on Palestinian Ma’an network, the other on BBC Arabic—explaining that the truce didn’t apply to areas of ongoing combat. They specifically named Shujaiyeh, where the market was hit, as a no-go zone.

It wasn’t immediately clear what hit the fruit and vegetable market and nearby fuel station, sending up a cloud of black smoke.



A Palestinian boy reacts as he sees the body of a relative who died in the Jabalia shelling. Hours after the blasts, the Israeli military declared a four-hour cease-fire in some areas of Gaza.
European Pressphoto Agency

Among the dead was Rami Rayan, a journalist with the Palestinian Network for Journalism and Media. The network said he had been at the market shooting video of shoppers.

A spokesman said the Israeli military was investigating the explosion.

By Wednesday evening, the military said it had hit 80 targets in the Gaza Strip during the day and militants had fired 140 rockets at Israel.

It said the military’s targets included five mosques used by Hamas to conceal weapons, tunnel access shafts and lookout posts. The military said it detonated three tunnel routes and hit a site being used by the head of Hamas’s cyber division.

Palestinian officials said the homes of several large families were destroyed early Wednesday. The Health Ministry said 10 members of the Al Astal family in Khan Younis were killed by a strike on their living room.

—Asa Fitch in Jerusalem and Joshua Mitnick in Tel Aviv contributed reporting.

Write to Nicholas Casey at nicholas.casey@wsj.com