"What's happening in Gaza is totally unacceptable," Guterres said in a post on social media platform X. "Six of our @UNRWA colleagues are among those killed."

UNRWA called it the highest death toll among its staff in a single incident.

"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 separately posted on X.

Earlier on Wednesday, the Israeli army and Gaza's civil defence agency said a strike hit Al-Jawni school in Nuseirat.

Civil defence agency spokesman Mahmud Bassal told AFP that 14 people were killed in the Israeli bombing.

The Israeli military said its air force had "conducted a precise strike on terrorists who were operating inside a Hamas command-and-control centre" on the school grounds, without elaborating on its outcome or the identities of those targeted.