The Marine Corps offered little information in a statement about the incident.
Six US marines were missing after 2 American military aircraft crashed on Thursday during a refueling operation off the coast of Japan, officials from both countries told AFP.
U.S. media reported five crew were on the C-130 and two on the F-18.
A Marine spokesman released a statement saying one of the personnel from the planes "is being evaluated by competent medical authorities".
A Facebook posting by the III Marine Expeditionary Force said the incident took place 200 miles (320km) off the coast.
Japanese search and rescue aircraft have responded to the incident.
The BBC's Rupert Wingfield-Hayes in Tokyo says that air-to-air refuelling is a hard and potentially risky flight operation, especially when done at night.
The FA-18 and the KC-130 from the Iwakuni base were conducting routine training when they collided some 100 kilometers south of Cape Muroto in Kochi Prefecture at around 1:40 a.m., according to the ministry.
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. The F/A-18 is a two-seater jet.
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