PHNOM PENH, Cambodia — A Cambodian military helicopter crashed during a training mission south of the capital on Monday, killing five people on board, police said.

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 six miles south of Phnom Penh. It crashed into a pond surrounded by rural land and rice fields, and had sunk into the mud, Tum Polin said.

Chhay Bunna, chief of police at the nearby international airport, said new air force pilots were undergoing training at the time.