Children were among the wounded in the school strike
Israel launched a deadly attack on a UN school housing refugees in Gaza despite repeated warnings that civilians were sheltering there, the UN has said.
UN spokesman Chris Gunness said "the world stands disgraced" by the attack, in which 15 were reported killed.
Israel, which has accused the militant group Hamas of using schools as bases to launch rockets, has not yet commented on the claims.
Some 1,200 Palestinians and 55 Israelis have been killed in the conflict.
Most of the Palestinian deaths have been civilians.
Some 53 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, 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.
A monthly opinion poll of about 600 Israeli Jews by Tel Aviv University suggests 97% support the current military operation.
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 were "multiple deaths" including women and children, adding that the attack caused "universal shame".
Images from the school showed large holes in the walls and roof.
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.
Israel has repeatedly accused Hamas of using schools and civilian areas as bases to launch attacks, although it is unclear whether the Abu Hussein school was under suspicion.
A UN-run school was hit last week, with Palestinians saying at least 15 were killed.
But the Israeli military "ruled out" that it was responsible in that case, saying a single "errant" shell had landed in an empty courtyard. It said it had come under fire from militants using anti-tank missiles in the vicinity of the school.
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.
But the organisation declined to give the location of the school or say who was responsible for the weapons.
In a statement, UNRWA said the rocket cache was "yet another flagrant violation of the neutrality of our premises. We call on all the warring parties to respect the inviolability of UN property."
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 infiltrate and 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.