Dozens of civilians were killed in an Israeli airstrike on a school run by the UN agency for Palestinian refugees (UNRWA) in central Gaza, Palestinian medics and the Hamas militant group said, a claim Israel denies.
The Hamas-controlled health authority in Gaza said that at least 30 people were killed in the attack on Wednesday night and that most of the victims were women and children. Hamas said 40 people died.
Israeli military spokesman Peter Lerner said Israel had no knowledge of possible civilian causalities.
Twenty to 30 members of Hamas and the Palestinian Islamic Jihad (PIJ) were believed to have been in three classrooms of the school building in the Nuseirat refugee neighbourhood, he said.
Initial findings indicated that many of them had been killed in the attack, Lerner said.
The school had been monitored in recent days and the attack had been postponed twice in order to avoid civilian casualties, the army spokesman added.
According to Palestinians, civilians fleeing the war from other areas of Gaza had sought refuge in the building. Hamas spoke of a "cruel massacre" of displaced Palestinians.
Lerner, however, said that internally displaced people had not been in the area attacked by Israel.
The United States called for an investigation into the airstrike and told Israel it must be "fully transparent."
US State Department spokesman Matthew Miller said that even if Israel's army intended to target Hamas militants, the claims of children killed in the strike "show that something went wrong."
"If that is accurate that 14 children were killed, those aren't terrorists," Miller said.
"These are all facts that need to be verified, and that's what we want to see happen."
It was impossible to independently verify the figures and allegations made by the sides.
The Israel Defense Forces (IDF) said earlier that militants who had participated in the October 7 massacre in Israel had been targeted at the UNRWA school.
Internally displaced Gazans have often taken refuge in UN school buildings since the start of the war, hoping Israel will not target UN buildings.
"Another horrific day in Gaza," wrote UNRWA chief Philippe Lazzarini on X. "The school was sheltering 6,000 displaced people when it was hit. Claims that armed groups may have been inside the shelter are shocking. We are however unable to verify these claims."
Since the beginning of the Gaza war, more than 180 UNRWA buildings have been hit, killing more than 450 displaced people, Lazzarini said.
According to the Israeli army, this is the fifth known case within a month in which Hamas and PIJ fighters have misused UNRWA facilities for their own purposes.
Israel accuses Hamas of operating from civilian facilities such as schools, hospitals and mosques, thereby endangering civilians or deliberately using them as shields.
Elsewhere, three Palestinians have been killed in clashes between Israeli soldiers and militants in the town of Jenin in the occupied West Bank.
Several other Palestinians were injured, the Palestinian Ministry of Health said.
According to the Israeli military, the soldiers arrested a wanted Palestinian. Militants then fired on them, which the troops returned. They received support from a combat helicopter during the battle. Several armed Palestinians were killed, it said.
Palestinian media reported, citing witnesses, that members of an undercover Israeli special forces unit had initially entered Jenin to arrest someone. Militants opened fire on the Israelis, whereupon more troops entered the town and engaged in skirmishes with the militants.
Jenin is considered a stronghold of Palestinian extremists.
Since the beginning of the war between Israel and Hamas in the Gaza Strip on October 7, the situation in the entire West Bank has deteriorated significantly.
According to the Ministry of Health, 509 Palestinians have since been killed in the West Bank in Israeli military operations, confrontations or their own attacks. There has also been an increase in settler violence against Palestinians.
Meanwhile, violence resumed on the border between Lebanon and Israel on Thursday.
On the Lebanese side, at least one fighter from the Lebanese Shiite militia Hezbollah was killed in Aitaroun near the border with Israel, local media reported. Another fighter was also injured.
The Israeli military said it had attacked Hezbollah infrastructure in the area.
Hezbollah again claimed several attacks on targets in northern Israel on Thursday.
The situation on the Israeli-Lebanese border has recently become significantly worse. According to the Israeli army, a soldier was killed in a drone attack from Lebanon on Wednesday. Hezbollah claimed responsibility for the attack.
UN Secretary-General António Guterres has renewed his calls for a ceasefire, his spokesman Stéphane Dujarric said in a statement.
"He remains gravely concerned that the exchanges of fire have not only ravaged communities close to the Blue Line but have also impacted deeper into the territories of both Lebanon and Israel, with the use of increasingly destructive weapons," he said.
"These exchanges of fire could trigger a broader conflict with devastating consequences for the region."
The United Nations supported all diplomatic and political efforts to end the violence, the statement added.
The Blue Line is a demarcation line between Israel and Lebanon set by the UN.
