More than 50 Palestinians were killed on Wednesday amid Israel's heaviest air and artillery bombardment in more than three weeks of fighting in Gaza.

The Israeli army has agreed to observe a temporary lull in fighting, a statement said.

"The IDF (army) has authorized a temporary window in the Gaza Strip. The window will commence today between 15:00-19:00 (1200-1600 UTC)," the statement said adding that the window would not apply to areas where ground troops were "currently operating."

Before dawn Israeli tank shells hit the UN school in Jebaliya refugee camp, UN Relief and Works Agency (UNRWA) spokesman Adnan Abu Hasna said.

UNRWA's director of northern Gaza operations, Khalil al-Halabi, said 3,000 Palestinians were taking refuge in the building at the time it came under fire.

"There were five shells - Israeli tank shells - which struck the people and killed many of them as they slept. Those people came to the school because it is a designated UN shelter," he said.

Abu Hasna called on the international community to step in following the attack that killed at least 16 people and wounded over 90 more.

"It's the responsibility of the world to tell us what we shall do with more than 200,000 people who are inside our schools, thinking that the UN flag will protect them," he said. "This incident today proves that no place is safe in Gaza."

UNRWA spokesman Chris Gunness said his organization condemned Israel's "violation of international law."

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

Deadly attacks widespread

Elsewhere Wednesday morning, at least six children were killed during Israeli attacks on Gaza, including an 11-year-old disabled girl who died during shelling on Gaza City and a 16-year-old girl killed in a strike on central Gaza, emergency services spokesman Ashraf al-Kidra said.

A single strike in the southern city of Khan Yunis killed ten members of the same family, including a child, al-Kidra said. Tank shelling also killed seven members of another family in a later incident in the same city.

Seven members of the same family, among them four children, were killed when an Israeli shell hit the northeastern Gaza City neighborhood of Tufah, al-Kidra said.

Wednesday's bloodshed brings Gaza's death toll to well over 1,200, with more than 7,100 Palestinians wounded, since Israel began its bombardment of the coastal enclave of 1.8 million people on July 8.

The number of Palestinians who have died soared since Israel began its Gaza ground offensive 10 days later. The army says its primary goal is to destroy Hamas' tunnel network along the border.

Fifty-three Israeli soldiers have been killed in that time period, along with two civilians and a Thai national.

Ceasefire calls

As the violence in Gaza continues, the international community has reiterated calls for an immediate ceasefire.

A Palestinian delegation was set to discuss a potential truce in Cairo, while Egypt said it was revising an unconditional ceasefire proposal originally accepted by Israel but rejected by Hamas.

The leader of Hamas' military wing, Mohammed Deif, said during a broadcast Tuesday that they would not accept a truce until Israel ended its blockade of Gaza, which is supported by neighboring Egypt.

German Foreign Minister Frank-Walter Steinmeier said Wednesday he was "extremely concerned about the escalating fighting in and around Gaza" in which civilian and UN buildings have been attacked.

"Everything must be done to prevent civilian victims and to uphold humanitarian law," he said. "I urge both sides to agree to an immediate ceasefire and to resume negotiations about a long-term ceasefire on the basis of the Egyptian suggestions."

dr/hc (AFP, Reuters, AP)