Israel bombed a school housing displaced Palestinians in central Gaza, which rescuers said killed 18 people, including UN staffers.
The Al-Jaouni school in Nuseirat had already been bombed several times over the course of the 11-month war in Gaza.
The strike on Wednesday flattened part of the UN-run facility where Gazans had sought shelter, leaving only a charred heap of rebar and concrete.
"For the fifth time, Israeli forces bombed the UNRWA-run Al-Jaouni School, killing 18 citizens, including two UNRWA staff members, children, and women, and injuring more than 18 others," Gaza's Civil Defence spokesperson Mahmud Bassal posted on Telegram, referring to the UN agency for Palestinian refugees.
UNRWA later said six of its staffers had been killed in two Israeli air raids on the Nuseirat school and its surroundings, calling it the highest death toll among its team in a single incident.
"Among those killed was the manager of the UNRWA shelter and other team members providing assistance to displaced people," the UN agency posted on X.
"Schools and other civilian infrastructure must be protected at all times, they are not a target."
Across the besieged strip, many school buildings have been repurposed to shelter displaced families as the vast majority of Gaza's 2.4 million people have been repeatedly uprooted by the war.
Israeli forces have struck several such schools in recent months, alleging that Palestinian fighters were operating there and hiding among displaced civilians - claims denied by Hamas.
Survivors of the strike scrambled to retrieve bodies and belongings from the rubble, telling AFP they had to step over "shredded limbs".
"I can hardly stand up," said one man, holding a plastic bag of human remains.
"We've been going through hell for 340 days now, what we've seen over these days, we haven't even seen it in Hollywood movies, now we're seeing it in Gaza."
Gaza has repeatedly been called the world's deadliest place for humanitarian workers.
UNRWA head Philippe Lazzarini said after the school strike that at least 220 members of the agency's staff have been killed in the Gaza war.
"Endless & senseless killing, day after day," he posted on X.
"Humanitarian staff, premises & operations have been blatantly & unabatedly disregarded since the beginning of the war."
UN chief Antonio Guterres called what is happening in Gaza "totally unacceptable".
In response, Israel's ambassador to the UN accused Guterres of distorting reality.
"It is unconscionable that the UN continues to condemn Israel in its just war against terrorists, while Hamas continues to use women and children as human shields," Ambassador Danny Danon wrote on social media.
"The solution," he added, "is not a ceasefire, but the release of all hostages still held in Gaza and the elimination of Hamas."
Further north in the Gaza Strip, the civil defence agency's spokesman Bassal said three people were killed overnight in Israeli shelling on the Jabalia refugee camp.
In Gaza City, he said there had been two deadly strikes in the Zeitoun neighbourhood - the first killing five including two children, and the second killing two and injuring several others.
Months of negotiations mediated by Qatar, Egypt and the United States have so far failed to secure a truce.
A Hamas delegation met Qatari and Egyptian mediators in Doha on Wednesday, the militant group said in a statement, though there was no indication of a breakthrough.
Recent rounds of mediation held in Doha and Cairo have tried to hash out a framework laid out in May by US President Joe Biden, but both Israel and Hamas have publicly signalled deeper entrenchment in their negotiating positions.
Israel's war on Gaza has killed at least 41,084 people in Gaza since 7 October. The UN rights office says most of the dead are women and children.
Israel's military meanwhile reported the deaths of two soldiers when a helicopter crashed in the area of Gaza's southern city of Rafah. Another eight soldiers were injured.
The aircraft was reportedly on a "life-saving operation" to evacuate a wounded soldier when it crashed, Major General Tomer Bar said in a Wednesday statement.
According to official figures, Israeli military's losses in the Gaza war stands at around 344.