A USA defense official told AFP there had been a crash after the plane took off from Marine Corps Air Station Iwakuni in southern Japan.
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. It was not clear how many crew members were on each of the planes.
The planes involved are a KC-130 and an F/A 18, carrying a total of seven personnel, U.S. media quoted officials as saying.
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The circumstances of the mishap were under investigation and no other information was available, according to the statement. It was unclear if US military officials were also assisting with the search.
The Marine Corps said Japanese search-and-rescue aircraft immediately responded to aid in recovery efforts.
Reports citing Marines officials indicate the incident involved a KC-130 refueling aircraft and a F/A-18 fighter jet, carrying 7 personnel in total.
More than 50,000 USA troops are based in Japan under the bilateral security pact.