Rescuers work near the helicopter in mountains in Nagano prefecture (Kyodo News/AP)

All nine people aboard a rescue helicopter that crashed in snow-covered mountains in central Japan have been pronounced dead.

The Fire and Disaster Management Agency said the nine men - seven firefighters, an engineer and the pilot - were aboard a rescue helicopter that crashed during a training flight on Sunday.

The helicopter, belonging to Nagano prefecture's rescue unit, crashed on the side of Mount Hachibuse, about 150 miles north west of Tokyo.

TV footage showed the mangled aircraft buried in deep snow, sitting belly up with its broken pieces scattered around.

The cause of the crash is under investigation.

Japanese public broadcaster NHK said the helicopter was a 15-seat Bell 420EP made by Bell Helicopter, based in Fort Worth, Texas.