Palestinian authorities said a grave site discovered at the Nasser hospital, the main medical facility in central Gaza, contained nearly 400 bodies
TASS news agency earlier said at least seven people were killed when the twin-engine L-410 crashed near the Tanay aerodrome, which provides parachuting services.
It also cited the local prosecutors' office as saying the crew had sent a distress signal about engine failure.
A medical services spokesman told Reuters the injured were being airlifted to hospitals.
Social media showed pictures of the heavily-damaged plane with an ambulance nearby.
A spokesman at a Siberian branch of Russian civil aviation agency Rosaviatsia said a search and rescue operation was under way, declining to provide details.