The Hamas-run government media office said 13 children, including a three-month-old baby, and six women were among the dead.
The Israeli military said it hit a Hamas command centre embedded in the compound that previously served as a school, repeating an accusation that Hamas uses civilian facilities for military purposes - a charge which Hamas denies.
In Rafah, in the southern Gaza Strip, the Gaza health ministry said four health workers were killed by an Israeli strike that hit ministry warehouses. Ambulance crews could not reach the dead or treat the wounded, it added.
In a statement on Saturday, the Israeli military said forces, operating in Rafah since May, have killed dozens of militants in recent weeks and dismantled military infrastructure and tunnel shafts.
Israel's demand to keep control of the southern border line between Rafah and Egypt has been a major sticking point in international efforts to conclude a ceasefire deal between Israel and Hamas.
Hamas says it is focused on an agreement to end the war and get Israeli forces out of Gaza, while Israel says the war can only end once Hamas is eradicated.
Another sticking point has been the specifics of an exchange of Israeli hostages for Palestinian prisoners held by Israel.
This war in the decades-old Israeli-Palestinian conflict was triggered on October 7 when Hamas attacked Israel, killing 1200 people and taking about 250 hostages, according to Israeli tallies.
Israel's subsequent assault on the Hamas-governed enclave has killed more than 41,000 Palestinians, according to the local health ministry, and displaced nearly the entire 2.3 million-strong population.
