The UN agency for Palestinian refugees (Unrwa) says six of its employees have been killed in an Israeli air strike on a school it runs in central Gaza.

Gaza's Hamas-run Civil Defence agency said a total of 18 people were killed in Wednesday’s strike on al-Jaouni school in Nuseirat refugee camp, which is being used as a shelter by thousands of displaced Palestinians.

Israel's military said it carried out a “precise strike on terrorists” planning attacks from the school, and that it had taken measures to avoid harm to civilians.

UN Secretary General António Guterres condemned the strike, saying: “What’s happening in Gaza is totally unacceptable.”

“These dramatic violations of international humanitarian law need to stop now,” he wrote on X, formerly Twitter.

Unrwa said the attack marked "the highest death toll among our staff in a single incident" since the start of the war between Israel and Hamas in October.

It also noted that it was the fifth time the school had been hit over the past 11 months.

In July, 16 people were reportedly killed in a strike which the Israeli military said had targeted several structures at the school used by Hamas fighters.

Israel's ambassador to the UN, Danny Danon, hit out at Guterres' criticism.

“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,” he said.

Hamas - which is proscribed as a terrorist group by Israel, the UK and other countries - has denied using schools and other civilian sites for military purposes.

Israeli forces launched a campaign to destroy Hamas in response to the group's unprecedented attack on southern Israel on 7 October, in which about 1,200 people were killed and 251 others were taken back to Gaza as hostages.

More than 41,080 people have been killed in Gaza since then, according to the territory's Hamas-run health ministry.