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.
Israel said an initial inquiry suggested soldiers responded to mortar fire. It called a partial, four-hour humanitarian ceasefire but Hamas, which controls Gaza, said it was meaningless.
More than 1,300 Palestinians and 55 Israelis have died 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 territory.
Hamas 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.
At the scene: BBC's Chris Morris in Gaza
It is a grim scene at the school: debris and personal belongings scattered among dried pools of blood, in a classroom where displaced Palestinians had been sleeping.
The UN says this is the sixth time that one of its schools has been hit by shellfire since this conflict began. And I've been told by UN officials on the ground that they believe Israeli forces were responsible on each occasion.
Israel often disputes the UN's version of events. When the UN said an Israeli mortar had hit a school in Beit Hanoun last week, for example, killing at least 13 people, Israel produced video from an aerial drone which it said showed the school compound was empty. But the UN told me - and it has made the same point to the Israeli military - that the resolution of the video is so poor compared with proper satellite imagery that you cannot see some of the trees in the compound, let alone the people.
Now the language the UN is using to protest against attacks on its schools is becoming stronger, as it has become angrier and more exasperated. Referring to today's school attack, the Commissioner-General of Unrwa, Pierre Krahenbuhl, could not have been any clearer: "I condemn in the strongest possible terms this serious violation of international law by Israeli forces."
Mr Gunness, from the UN Relief and Works Agency (Unrwa), told the BBC that Israel had been told 17 times that the school in the Jabaliya refugee camp was housing the displaced.
"The last time was hours before the fatal attack," he said. "Our initial assessment is that it was Israeli artillery that hit our school."
He said there had been "multiple deaths" including women and children, adding that the attack caused "universal shame".
Bob Turner, Unrwa's Gaza director, said the UN was "confident" Israel was responsible.
He said UN workers had collected fragments of projectiles that suggested they were artillery shells fired from Israeli positions to the north-east of the school.
The Israeli military said in a statement that its "initial inquiry suggests militants fired mortars earlier this morning from the vicinity of Unrwa school in Jabaliya".
It said soldiers had "responded by firing towards the origin of fire".
Israel has repeatedly accused Hamas of using schools and civilian areas as bases to launch attacks.
The military 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.
Sirens continued to sound in southern Israel to warn of militant rocket attacks and Palestinian doctors said an Israeli air strike after the truce was announced killed seven people in Khan Younis.
Later, a Palestinian official said 15 people had been killed and 150 wounded in an air strike on a market in Gaza.
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.
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.