Fishing boats in the area found three people in the surrounding waters, a representative of a local fisheries cooperative said, adding their condition was unknown.
Earlier reports claimed there were eight crew members aboard the aircraft, but that has since been revised to six.
Another Osprey landed safely at the island’s airport on Wednesday afternoon around the time of the crash, a spokesperson for the local government said.
US forces in the region were still gathering information, a spokesperson said.
The crash happened just before 3pm with witnesses saying the aircraft’s left engine appeared to be on fire as it descended, media reported.
Japan, which also operates Osprey aircraft, said on Wednesday it had no plans to ground the aircraft but had asked the US military to investigate the crash.
Developed jointly by Boeing and Bell Helicopter, the Osprey can fly both like a helicopter and fixed-wing aircraft and is operated by the US Marines, US Navy and the Japan Self Defense Forces.
The deployment of the Osprey in Japan has been controversial, with critics saying it is prone to accidents. The US military and Japan say it is safe.
Another crash-landed in the ocean off Japan’s southern island of Okinawa in December 2016, prompting a temporary US military grounding of the aircraft.
Additional reporting by Agence France-Presse