Israeli airstrikes hit a school that was sheltering families in central Gaza, killing six of its employees, said the United Nations Relief and Works Agency for Palestinian refugees (UNRWA) in the Gaza Strip.

UNRWA said this is the fifth time the al-Jaouni school has been hit since the start of the war. Some 12,000 people — mostly women and children — are taking shelter there and the manager of the UNWRA team helping people was killed in Wednesday's strike.

First responders said 18 people died in the attack that caused the roof of a shelter to cave in. Families were left scrambling to find their loved ones amid the wreckage.

“No one is safe in Gaza,” the agency said in a statement. “No one is spared.”

António Guterres, the U.N. Secretary General, called the strikes “dramatic violations of international humanitarian law."

“What’s happening in Gaza is totally unacceptable,” he wrote on X, formerly Twitter.

The Israeli military said Hamas was using the school as a “command and control center."

UNWRA called on all parties in the conflict to “never use schools or the areas around them for military or fighting purposes” and said that “schools and other civilian infrastructure must be protected at all times."

Sixteen people were reported killed in a previous Israeli attack on several structures in the Al-Jaouni school compound in July, which Israel said was being used by Hamas fighters.

UNRWA says Wednesday's attack caused the highest number of staff killed in a single incident. At least 220 agency staff have lost their lives since the war began. Philippe Lazzarini, the UNRWA Commissioner-General, said humanitarian staff are being “disregarded” in the Gaza war.

“The longer impunity prevails, the more international humanitarian law and the Geneva conventions will become irrelevant,” he wrote on social media.

Gaza’s Health Ministry said more than 41,000 Palestinians have been killed and 95,125 injured in Israel's military offensive on Gaza since Oct. 7, the day that Hamas attacked Israel, killing 1,200 people and taking 250 hostages.

Millions of Gaza residents have been displaced in the war, many of them multiple times. The population is blighted by hunger and the spread of disease.