At least 23 people were killed under 24 hours as Israel targeted four schools being used as shelters in the Gaza Strip, the United Nations agency for Palestinian refugees (UNRWA) said late Thursday.

The four "schools-turned-shelters" have served as temporary homes for some 20,000 people, who have been displaced by the nearly four weeks of relentless attacks by Israeli forces on the densely populated Palestinian enclave.

At least 20 people were killed and five were wounded at a shelter school at the Jabalia refugee camp, the largest in the Gaza Strip, two days after massive bombardments in the area, UNRWA Commissioner-General Philippe Lazzarini said in a news release.

At least 195 people were killed in Jabalia on Tuesday and Wednesday, according to the Gazan media office.

Earlier Thursday, another shelter school in the north of the Gaza Strip was also damaged. One child was reportedly killed.

Further south, two schools-turned-shelters in the al Bureij Refugee Camp were also hit, the UNRWA said. Two people were reportedly killed and 31 were wounded.

"Since the start of the war on Oct. 7, nearly 50 UNRWA buildings and assets have been impacted, with some being directly hit," Lazzarini said.

Those buildings are being used as shelters and currently hosting around 700,000 people.

One of the victims was a young Palestinian refugee, who was also a UNRWA staff member, named Mai Ibaid.

She was "a bright software developer in her mid-20s with physical disabilities," Lazzarini said. "She was displaced from her home and killed in the Jabalia Refugee Camp with members of her family."

Ibaid was one of 72 UNRWA staffers killed since Oct. 7, marking the highest number of U.N. aid workers killed in a conflict in such a short time, the agency said.

"How many more?" Lazzarini wrote. "How much more grief and suffering? A humanitarian cease-fire is overdue for the sake of humanity."