The incident in Gaza City highlights the ongoing danger for tens of thousands of Palestinians who live in an estimated 2000 damaged buildings due to Israeli air strikes across Gaza.
Rescue workers from the civil defence said at least 12 people, including five children, were killed and 14 others injured overnight when the seven-storey building collapsed.
Almost 100 more people were believed to be trapped under the rubble.
A video obtained by Reuters showed rescue workers trying to pull out two people from under a collapsed wall.
Another video circulated on social media showed Gaza's civil defence spokesman, Mahmoud Basal, carrying a little girl, wrapped in a blood-stained scarf, in his hands.
Raed Al-Dahshan, the director of Gaza City's civil defence service, said at least 10 families lived in the collapsed building and there was still hope of finding survivors.
"We are still hearing voices from under the rubble, so there is still hope to find people," Dahshan said.
Alessandro Mrakic, head of the UN Development Programme's Gaza office, said rescuers from some UN agencies were taking part in the rescue efforts, which he described as difficult due to the lack of proper machinery.
"The situation, as you see, is tragic because we don't have the equipment and machinery that is needed. So there are actually people that are digging with their bare hands. We need to prepare (for more deaths)," Mrakic said.
"There are approximately 400 buildings at risk of collapse. And we know that these buildings will collapse, especially now with the upcoming winter."
Palestinian and UN officials blame Israel's restrictions on Gaza for the lack of heavy machinery in the territory.
Israel says its curbs aim to keep equipment out of the hands of militant groups.
Israel's assault on Gaza - launched after Hamas' deadly raid into Israel in October 2023 - has generated about 61 million tonnes of wreckage and debris that could take seven years to clear, according to the United Nations.
Dahshan warned residents to stay away from wrecked buildings due to the risk of collapse.
The local rescue service says about 2000 damaged buildings are inhabited by families because of the limited availability of shelter options.
Many families have told Reuters in the past months that they resorted to living in partially damaged buildings to protect themselves against wind and rainstorms, and because of the lack of new tents for the homeless.
An October 2025, US-backed ceasefire halted large-scale fighting in Gaza, but it has not ended Israeli strikes and there has been little progress on steps towards permanent peace.
Hamas accuses Israel of violating the ceasefire and undermining efforts to implement US President Donald Trump's broader plan to end the Gaza conflict, which includes an Israeli withdrawal from the Gaza Strip and the disarmament of Hamas.
Israel says Hamas has been violating the deal.
More than 1300 Palestinians, mostly civilians, have been killed in Gaza since the ceasefire took effect, according to the territory's health officials, while the Israeli military says four Israeli soldiers have been killed.
Hamas does not usually disclose information about fatalities among its fighters.
