The Japan Coast Guard received an emergency call about the crash at 2.47 p.m. after the Osprey departed from the Iwakuni US base in the Yamaguchi heading for the Kadena base in Okinawa, public broadcaster NHK said, citing local authorities.
The aircraft disappeared from radar at 2.40 p.m., Xinhua news agency quoted the Defence Ministry as saying.
Japan Coast Guard spokesperson Kazuo Ogawa said the coast guard has dispatched patrol boats and aircraft to the site for search and rescue operations but it had no further details of the crash, including the safety of those onboard.
Weather officials said the skies over the Yakushima area were mostly clear during the day, and the wind speed at 2.30 p.m. was 6.84 km per hour. No advisories or warnings were issued at the time.
The tilt-rotor Osprey aircraft, which can fly both like a helicopter and fixed-wing aircraft, has a track record of accidents and mishaps both in Japan and abroad.
In August, a US Osprey crashed off the coast of northern Australia, killing three crew members.
Another crash-landed in the ocean off Japan’s southern island of Okinawa in December 2016.