Israel attacked a UN-run school housing refugees in Gaza despite warnings that civilians were there, the UN has said.
UN spokesman Chris Gunness said “the world stands disgraced” by the attack, in which 15 died and dozens were hurt.
The Israeli military said an initial inquiry suggested soldiers responded to mortar fire. The military says it is now holding a partial, four-hour humanitarian ceasefire.
Some 1,200 Palestinians and 55 Israelis have been killed in the conflict.
Most of the Palestinian deaths have been civilians.
Fifty-three Israeli soldiers have been killed along with two civilians. A Thai worker in Israel has also died.
Israel launched its offensive in Gaza after a surge in rocket fire from the territor
Hamas, which controls Gaza, says it will not stop fighting until the blockade, maintained by both Israel and Egypt, is lifted.
The current conflict is now the longest between Israel and militants from Gaza.
A 2012 offensive lasted for eight days, and the 2008 conflict went on for 22 days.
The Israeli military said the incident was under review.
It said in a statement that its “initial inquiry suggests that militants fired mortars earlier this morning from the vicinity of Unrwa school in Jabaliya”.
It said soldiers “responded by firing towards the origin of fire”.
The military later said a ceasefire would be in force between 15:00 (12:00 GMT) and 19:00.
However, it would only apply to areas where Israeli soldiers were not currently operating and residents were warned not to return to areas they had previously been asked to evacuate.
Lt Col Peter Lerner of the IDF told the BBC: “I hope that Hamas will hold their fire as well, because otherwise things are going to get messier.”
Hamas spokesman Sami Abu Zuhri said the truce was meaningless.
“The lull which Israel announced is media exploitation and has no value because it excludes the volatile areas along the border, and we won’t be able to get the wounded out from those areas,” he said in a statement reported by Agence France-Presse.
Israel has repeatedly accused Hamas of using schools and civilian areas as bases to launch attacks.
Last week, another UN-run school was hit, with Palestinians saying at least 15 people were killed.
But the Israeli military denied the killings, saying a single “errant” shell had landed in an empty courtyard.
In other developments:
- The UN on Tuesday revealed that a cache of rockets had been found at one of its schools in Gaza – the third case of its kind – and condemned it as a “yet another flagrant violation of the neutrality of our premises”
- A monthly opinion poll of about 600 Israeli Jews by Tel Aviv University suggests 97% support the current military operation
- A baby who was born after her mother was killed in Gaza, making headlines around the world, has died
Israel stepped up the intensity of its strikes on Tuesday and overnight into Wednesday, saying it had hit a number of tunnels dug by militants to attack Israel.
But the Israeli military said rockets continued to land in Israel from Gaza.
Palestinian officials said Gaza’s port had been destroyed on Tuesday and its only power plant had been put out of action.
Meanwhile, Palestinian factions Hamas, Fatah and Islamic Jihad are expected to meet in Cairo later to discuss a ceasefire.