It was not immediately clear what caused the accident.
At least one person survived but was seriously injured, said Tum Polin, a police officer at the crash site.
The chopper went down about 10 kilometres south of Phnom Penh. It crashed into a pond surrounded by rural land and rice fields, and had sunk into the mud, Mr Tum said.
Chhay Bunna, chief of police at the nearby international airport, said new air force pilots were undergoing training at the time.