2 Marine Corps aircraft were involved in a 'mishap' over Japan, and a search-and-rescue operation is underway

Search-and-rescue operations are underway for two US Marine Corps aircraft involved in a "mishap" off the coast of Japan, the Marine Corps said Wednesday.

The service did not say which aircraft were involved or describe the nature of the incident. Media reports indicated the planes in question were an F/A-18 fighter and a KC-130 cargo plane and that they were taking part in aerial refueling operations.

"The aircraft involved in the mishap had launched from Marine Corps Air Station Iwakuni and were conducting regularly scheduled training when the mishap occurred," the service said in a statement published by USNI News.

The incident happened around 2 a.m. local time on December 6. Japanese search-and-rescue aircraft responded immediately, the Corps said, adding that the circumstances were under investigation.

The C-130 had five crew aboard and the F-18 two; their condition was not immediately known, Mary Walsh of CBS News reported.