In a joint statement, UN secretary-general Ban Ki-moon and US secretary of state John Kerry have announced a temporary break in the fighting between Israel and Palestinian group Hamas, which is due to start 8:00am Friday local time (3:00pm AEST).

The statement said “forces on the ground will remain in place” during the truce, implying that Israeli ground forces will not withdraw.

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“We urge all parties to act with restraint until this humanitarian ceasefire begins, and to fully abide by their commitments during the ceasefire,” Mr Kerry and Mr Ban said.

“This ceasefire is critical to giving innocent civilians a much-needed reprieve from violence … and the opportunity to carry out vital functions, including burying the dead, taking care of the injured and restocking food supplies.”

Israeli and Palestinian delegations will travel to Cairo immediately, at the invitation of Egypt, in a bid to negotiate a more durable end to the fighting.

The announcement came after the US government denounced the shelling of a United Nations school in Gaza that killed 15 people as “totally unacceptable and totally indefensible”.

Israel is being widely blamed for the strike on the school in the Jabaliya refugee camp, which occurred despite warnings that the school was being used as a civilian shelter.

In the strongest criticism of the Gaza offensive yet from the US, Israel’s closest ally, White House spokesman Josh Earnest said Israel’s government and military are not doing enough to spare civilian lives.

“The shelling of a UN facility that is housing innocent civilians who are fleeing violence is totally unacceptable,” he said.

The US had earlier refused to apportion blame for the attack.

More than 3,000 people, including many women and children, had been sheltering at the Abu Hussein School when it was attacked.

Israel says it is investigating the strike.

Gaza health officials say more than 1,400 Palestinians, most of them civilians, have been killed in the battered enclave since conflict flared more than three weeks ago.

Israel says 56 of its soldiers and three civilians have been killed by rockets fired by the Hamas militant group.

Mr Earnest urged Israel to end its ground operation in Gaza, saying: “These reports that hundreds of innocent Palestinian civilians have been killed are tragic.”