The UN chief condemned an Israeli strike on a UN-run school that the Israeli military alleged housed a "Hamas compound" as Gaza medics said at least 40 people had been killed.
"It's just another horrific example of the price that civilians are paying, that Palestinian men, women and children who are just trying to survive (are paying)," UN Secretary-General Antonio Guterres' spokesman Stéphane Dujarric said.
"Of course he condemns this attack. There will need to be accountability for everything that has happened in Gaza."
The United States has called on Israel to be transparent over the strike - including on whether children were killed.
"The government of Israel has said that they are going to release more information about this strike, including the names of those who died in it. We expect them to be fully transparent in making that information public," State Department spokesman Matthew Miller told reporters.
Mr Miller said the US believed Israeli assessments that Hamas has at times hidden in civilian infrastructure, but said it was waiting for information on the latest strike.
"We've seen the claims that 14 children were killed in this strike, and certainly, when you see - if that is accurate - that 14 children were killed, those aren't terrorists," he said.
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Earlier, European Union foreign policy chief Josep Borrell called for a probe into the Israeli strike.
"Reports coming from Gaza time and again show that violence and suffering are still the only reality for hundreds of thousands of innocent civilians. This appalling news must be independently investigated," Mr Borrell wrote on X.
Israel hit the school with what it described as a targeted airstrike on up to 30 Hamas fighters inside, but a Palestinian official said 40 people including women and children were killed as they sheltered in the UN site.
Video footage showed Palestinians hauling away bodies after the attack, which took place at a sensitive moment in mediated talks on a ceasefire that would involve releasing hostages held by Hamas and some of the Palestinians held in Israeli jails.
Ismail Al-Thawabta, the director of the Gaza government media office, rejected Israel's assertion that the UN school in Nuseirat, in central Gaza, had hidden a Hamas command post.
"The occupation uses ... false fabricated stories to justify the brutal crime it conducted against dozens of displaced people," Mr Thawabta told Reuters.
Israel's military claimed its fighter jets had carried out a "precise strike" and circulated satellite photos highlighting two parts of a building where it said the fighters were based.
'Why bomb us' - Gazans shocked after Israel hits UN school
"We're very confident in the intelligence," military spokesperson Lt Col Peter Lerner claimed, accusing Hamas and Islamic Jihad fighters of deliberately using UN facilities as operational bases.
He claimed 20-30 fighters were located in the compound, and many of them had been killed, but had no precise details as intelligence assessments were being carried out.
"I'm not aware of any civilian casualties and I'd be very, very cautious of accepting anything that Hamas puts out," he said.
As people at the school cleared rubble from bloodstained classrooms, survivor Huda Abu Dhaher described waking up to the sound of rockets.
"People's remains were scattered inside the yard and outside. The gas canister exploded," she told Reuters.
"My nephew was martyred (killed), he lost his leg and arm, he was a 10-year-old ... This woman's leg got a fragment in it, her son bled from his mouth and leg, her mother-in-law sustained three injuries."
The school, run by the UN Palestinian refugee agency (UNRWA), was sheltering 6,000 displaced people at the time, UNRWA chief Philippe Lazzarini said.
"At least 35 people were killed and many more injured," he wrote on X.
"Claims that armed groups may have been inside the shelter are shocking. We are however unable to verify these claims.
"Attacking, targeting or using UN buildings for military purposes are a blatant disregard of International Humanitarian law."
Mr Thawabta and a medical source said 40 had been killed, including 14 children and nine women.
Ceasefire efforts
Israel announced a new military campaign in central Gaza as it battles fighters relying on hit-and-run insurgency tactics.
It said there will be no halt to fighting during the ceasefire talks, which have intensified since US President Joe Biden outlined a proposal on Friday.
"At this decisive moment, we call on the leaders of Israel as well as Hamas to make whatever final compromises are necessary to close this deal," said the statement issued by the White House jointly with Argentina, Austria, Brazil, Britain, Canada and others.
Hamas seeks a permanent end to the war. Israel said it must destroy the militant group first.
In another sensitive development, the Israeli military reported a rare attack near the Israel-Gaza border, saying a squad of Palestinian fighters killed a soldier and three of them were killed in return fire.
A statement by Hamas armed wing, the Al-Qassam Brigades, said its fighters had conducted an operation behind enemy lines in the Rafah area of southern Gaza, a location corresponding to the Israeli military's account.
The incident was likely to challenge efforts to restore a sense of security in Israeli border communities that were overrun by Hamas-led gunmen on 7 October last year.
CIA director William Burns met senior officials from mediators Qatar and Egypt in Doha on to discuss the ceasefire plan.
Two Egyptian security sources said talks continued but had shown no sign of breakthrough.
Mr Biden has repeatedly declared that ceasefires were close over the past several months, but there has been only one, week-long truce, in November.
Last week's announcement coincides with intense domestic political pressure on Israeli Prime Minister Benjamin Netanyahu to chart a path to end the eight-month-old war and negotiate the release of hostages held by Hamas.
A centrist party in his emergency coalition government has threatened to quit by Saturday if he does not commit to a post-war plan for Gaza.
Far-right members have pledged to quit if he agrees to a peace deal that leaves Hamas in place.
The war was sparked by Hamas' 7 October attack on Israel, which resulted in the deaths of 1,194 people, mostly civilians, according to an AFP tally based on Israeli official figures.
Around 250 hostages were captured by Hamas, according to Israeli tallies. About half of them were freed in the November truce.
Israel's retaliatory war has killed at least 36,654 people in Gaza, also mostly civilians, according to the territory's health ministry.
US and Israeli officials have told Reuters about half of Hamas's forces have been killed in the conflict. Hamas does not disclose fatalities among its fighters and some officials said Israel exaggerated the figures.
Israel's own military death toll is almost 300.
