At least 30 people sheltering at a girls' school in Deir Al-Balah were taken to Al Aqsa Hospital and pronounced dead after a strike on Saturday that Israel's military said targeted a Hamas command and control centre used to store weapons and plan attacks.
It said militants "used the compound as a hiding place to direct and plan numerous attacks against IDF troops" and "developed and stored large quantities of weapons inside".
Civil defence workers in Gaza said thousands were sheltering in the school, which also contained a medical site.
Gaza's health ministry said at least 12 people had been killed in other strikes on Saturday.
The strikes hit a day before officials from the US, Egypt, Qatar and Israel are scheduled to meet in Italy to discuss the ongoing hostage and ceasefire negotiations.
CIA Director Bill Burns is expected to meet Qatari Prime Minister Mohammed Bin Abdul Rahman al-Thani, Mossad director David Barnea and Egyptian spy chief Abbas Kamel on Sunday, according to officials from the US and Egypt who spoke on condition of anonymity as they were not authorised to discuss the plans.
Injured people are taken to hospital after an Israeli bombing east of the Khan Younis camp. (EPA PHOTO)
US officials said on Friday that Israel and Hamas were in agreement on the basic framework of the three-phase deal under consideration.
However, in Israeli Prime Minister Benjamin Netanyahu's speech to the US Congress, he vowed to press on with the war until Israel achieved "total victory".
Palestinian officials vehemently condemned the speech after the school strike.
Nabil Abu Rudeineh, spokesman for Palestinian Authority President Mahmoud Abbas, said Netanyahu's reception from his supporters in the United States constituted a "green light" to continue Israel's offensive.
Earlier, Israel's military ordered the evacuation of a part of a designated humanitarian zone in Gaza before a planned strike on Khan Younis on Saturday.
The evacuation order was in response to rocket fire that Israel said originated from the area.
The military said it planned an operation against Hamas militants in the city, including parts of Muwasi, the crowded tent camp in an area where Israel has told thousands of Palestinians to seek refuge throughout the war.
It's the second evacuation order issued in a week that has included striking part of the humanitarian zone, a 60-square-kilometre area blanketed with tent camps that lack sanitation and medical facilities and have limited access to aid, United Nations and humanitarian groups say.
Israel expanded the zone in May to take in people fleeing Rafah, where more than half of Gaza's population at the time had crowded.
According to Israeli estimates, about 1.8 million Palestinians are sheltering there after being uprooted multiple times in search of safety during Israel's punishing air and ground campaign.
In November, the military said the area could still be struck and that it was "not a safe zone, but it is a safer place than any other" in Gaza.
Further north, Palestinians mourned the deaths of seven killed by Israeli air strikes overnight on Zawaida, in central Gaza.
Deir al-Balah's Al Aqsa Hospital confirmed the count and Associated Press journalists saw the bodies.
The war in Gaza has killed more than 39,200 Palestinians, according to the territory's health ministry, which does not distinguish between combatants and civilians in its count.
The war began with an assault by Hamas militants on southern Israel on October 7 that killed 1200 people, most of them civilians, and took about 250 hostages.
About 115 are still in Gaza, about a third of them believed to be dead, according to Israeli authorities.
