The airstrike, which also killed women and children, raised the death toll of staff from UNRWA, the UN agency helping Palestinian refugees in Gaza, to 220, spokesman Stephane Dujarric said Thursday.
The Israeli Defence Forces said they had targeted a Hamas command-and-control centre in the Nuseirat refugee camp in central Gaza.
The secretary-general Antonio Guterres believes "the continued lack of effective protection for civilians in Gaza is unconscionable," Dujarric said.
"Civilians and the infrastructure they rely on must be protected and meet the essential needs of the civilian population," the spokesman said.
"The secretary-general calls upon all parties to refrain from using schools, from using shelters, and the areas around them for military purposes," Dujarric said. "All parties to the conflict have the obligation to comply with international humanitarian law at all times."
Guterres reiterated his call for an immediate ceasefire and release of all hostages, the spokesman said, adding "this horrific violence must stop."
Hamas-led militants killed some 1200 people, mostly Israeli civilians, in their October 7 attack that sparked the war. They abducted another 250 and are still holding around 100. Around a third of them are believed to be dead.
More than 41,000 Palestinians have been killed in Gaza since the Israel-Hamas war began according to health officials, and about 90 per cent of Gaza's population of 2.3 million have been displaced, often multiple times.
Meanwhile, Turkey's justice minister says his country is investigating the death of a Turkish American activist shot and killed by Israeli forces last week while protesting settlements in the occupied West Bank.
The 26-year-old activist from Seattle was taking part in a demonstration against settlements in the Palestinian territory when she was fatally shot last Friday. Israel is investigating the killing of Aysenur Ezgi Eygi and its military later said she was likely shot "indirectly and unintentionally" by soldiers.
Turkish Justice Minister Yilmaz Tunc said Turkey would present its findings to a UN court overseeing a genocide case against Israel filed by South Africa over the war in Gaza.
"We will take every judicial step for our martyred daughter, Aysenur," Tunc said.
The activist's uncle said the United States needs to protect the rights of its citizens everywhere.
Ali Tikkin welcomed Vice President Kamala Harris's remarks that the activist's death was unacceptable and demanded "complete accountability" for it.
The US should "protect the rights of its citizens and follow them to the end," said Tikkin.
Within Israel, medics say four people were killed and eight others injured when a car exploded, as gang violence surges within Israel's Arab communities.
The deadly car blast in the city of Ramle set off a wave of anger against Israeli security forces, whom residents accuse of failing to fight the escalating internal gang wars and gun violence affecting Palestinian citizens of Israel.
Israel's far-right national security minister, Itamar Ben-Gvir, arrived at the crime scene to speak with residents, but was drowned out by angry hecklers. Scuffles between victims' family members and police erupted outside the hospital where the injured were being treated
