Japanese and US officials say two American warplanes crashed into the Pacific Ocean off Japan's southwestern coast after a midair collision early Thursday, and search and rescue operations are underway.
Search and rescue operations were underway following a "mishap".
It was unclear if U.S. military officials were also assisting with the search.
The aircraft were identified as a KC-130-a plane involved in aerial refueling-and an F/A-18 Hornet-a twin engine fighter jet.
The Marine Corps tweeted that the incident occurred at about 02:00 on Thursday (17:00 GMT Wednesday).
A Facebook posting by the III Marine Expeditionary Force said the incident took place 200 miles (320km) off the coast.
Japanese aircraft were taking part in the search effort, according to the Marines.
The planes took off from Marine Corps Air Station Iwakuni in southern Japan and were "conducting regularly scheduled training when the mishap occurred", the statement said.
The survivor is being assessed by doctors at MCAS Iwakuni.
They said Japanese rescuers found one of the crew members who is in a stable condition.
A US Marine Corps F-18 fighter takes off from the North Atlantic Treaty Organisation military base in Aviano, Italy.