A strike this weekend on a school run by the United Nations that was being used for shelter by thousands of displaced people in northern Gaza killed at least 24 people, a U.N. official said Sunday,

Meanwhile, Israel issued a new warning to residents of Gaza’s largest refugee camp Jabalia and a nearby coastal camp to evacuate, as the military said Sunday it was “expanding its operational activities in additional neighbourhoods… of the Gaza Strip.”

A Palestinian health official said that according to their estimates, more than 80 people were killed in twin strikes on Jabalia on Saturday, including on a UN school sheltering displaced people.

In his remarks, Lazzarini, the UNRWA chief, said that another school, where some 4,000 displaced people were sheltering at the time, in Gaza City, was “directly struck” on Friday, and ambulances “could not reach the school to provide help.” UNRWA said it believed that scores of people had been either killed or wounded, but did not have exact numbers.

UN Secretary-General Antonio Guterres, said that hundreds of thousands of Palestinian civilians have sought shelters at U.N. facilities throughout Gaza. “I reaffirm that our premises are inviolable,” he said in a statement on Sunday.

At least 176 displaced people sheltering at schools run by UNRWA have been reported killed and nearly 800 wounded since the start of Israel’s bombing campaign against Gaza on Oct. 7, according to the UNRWA statement. Classes have been suspended and the schools have been converted into shelters across Gaza.

The UN agency said that it was still verifying its figures for the numbers of dead and wounded, but that it had confirmed that at least 17 of its facilities have been directly hit.

The Al-Fakhura school was also hit by a strike on Nov. 4, killing at least 12 people and wounding 54 others, UNRWA said.

According to UNRWA, almost 900,000 displaced people are sheltering in the agency’s facilities across the Gaza Strip. They include many people who fled northern Gaza following Israeli orders to evacuate the area amid an escalation in ground fighting there.