The U.N. Palestinian refugee agency UNRWA said six staffers were killed after two airstrikes hit a school in central Gaza on Wednesday, marking what it said was the highest death toll among its staff in a single incident.

“Among those killed was the manager of the UNRWA shelter and other team members providing assistance to displaced people,” UNRWA said on X.

Earlier on Wednesday, the Israeli army said in a statement that it conducted a strike on a command and control center in Nuseirat in central Gaza, which it said was operated by Palestinian militant faction Hamas.

“This school has been hit five times since the war began. It is home to around 12,000 displaced people, mainly women and children,” UNRWA added.

The Hamas-run government media office said the Israeli strike killed at least 18 people, including the UNRWA staff members.

UN Secretary-General Antonio Guterres deplored the killings and said these “dramatic violations of international humanitarian law” needs to stop.

“What’s happening in Gaza is totally unacceptable. A school turned shelter for around 12,000 people was hit by Israeli airstrikes again today. Six of our @UNRWA colleagues are among those killed. These dramatic violations of international humanitarian law need to stop now,” the UN chief said in a post on X.

The Israeli military says it takes steps to reduce the risk of harm to civilians and that at least a third of the Palestinian fatalities in Gaza are militants. It accuses Hamas of using Palestinian civilians as human shields, which Hamas denies.

The war was triggered on Oct. 7 when Hamas attacked Israel, killing 1,200 people and taking about 250 hostages, according to Israeli tallies. Israel’s subsequent assault on Gaza has killed more than 41,000 Palestinians, according to the enclave’s health ministry.

(With inputs from agencies)