Palestinian authorities said a grave site discovered at the Nasser hospital, the main medical facility in central Gaza, contained nearly 400 bodies
The bodies of three adults and two children have been found in the rubble so far.
Up to eight people could have been in the building and firemen are continuing to search for other victims, a local police spokesman said.
Up to 100 firemen and 50 police officers are on the scene, Polish state broadcaster TVP reported.