Japanese officials said two crew members were on the FA-18, and five others on the KC-130.
Some media reports said the planes had taken off from Marine Corps Air Station Iwakuni, which is near Hiroshima in the south-west of the country and that the incident happened off the coast.
Reports on Twitter indicate that two US military aircraft have crashed during a midair refueling operation off Iwakuni, Japan.
Search and rescue operations were underway following a "mishap".
In a statement, the Marine Corps said the planes had taken off from Marine Corps Air Station Iwakuni in southern Japan.
Luke MilanoUS Marines Launch Search and Rescue Effort After Two Planes Crash Near Japan
The Marines said in a statement that Japanese search and rescue aircraft responded and were aiding in the rescue effort. Very few details have been made public in the initial Corps news release.
A Marine Corps KC-130 aerial tanker and an F/A-18 Hornet have crashed off Japan, a defense official told Task & Purpose on Wednesday.
The Marine Corps said in a short statement that the incident occurred at 2 a.m. local time in Japan on Thursday (1700 GMT Wednesday).
UPDATE: This story was updated at 5:45 P.M. on December 5 after one of the missing Marines was found alive.
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