A Greek Orthodox church in the Gaza Strip which was sheltering hundreds of displaced Palestinians was hit overnight by an Israeli air strike, the Orthodox Patriarchate of Jerusalem and Palestinian health officials said.
Gaza's Hamas-run government media office said 18 Christian Palestinians were killed. There was no word from the church on any final death toll.
The Israeli military said a part of the church was damaged in a strike on a militant command centre and it was reviewing the incident.
Palestinian officials said at least 500 Muslims and Christians had taken shelter in the church from Israeli bombardments.
The Orthodox Church said in a statement: "The Orthodox Patriarchate of Jerusalem expresses its strongest condemnation of the Israeli air strike that has struck its church compound in the city of Gaza."
Video from the scene at the church compound showed a wounded boy being carried from rubble in the nighttime. A civil defence worker said two people on upper floors had survived. Those on lower floors had been killed and were still in the rubble, the worker said.
Gaza's 2.3 million population comprises an estimated 1,000 Christians, most of whom are Greek Orthodox.
The Israeli military said its fighter jets had hit a nearby command and control centre that was used to carry out attacks against Israel.
"As a result of the IDF strike, a wall of a church in the area was damaged. We are aware of reports on casualties. The incident is under review," it said.
Israel has pounded densely-populated Gaza, flattening buildings and destroying infrastructure, since Hamas attacked southern Israel on Oct. 7 and killed 1,400 people.
Nearly 3,800 people in Gaza have since been killed by Israeli strikes and more than a million have been made homeless, according to Palestinian health officials. Civilians say their situation is desperate as they run short of food, water, fuel and medical supplies.
Israel appears to be getting closer to a full-scale invasion of Gaza. The Israeli military has massed troops and equipment near the Gaza border.