There is no official word on where the incident took place.
The aircraft - a KC-130 and an F/A-18 Hornet - had launched from Marine Corps Air Station Iwakuni in southern Japan. "Japanese search and rescue aircraft immediately responded to aid in recovery".
Japan's Defense Ministry said the aircraft collided midair and crashed into the Pacific Ocean about 60 miles south of Muroto Cape on Shikoku Island, according to The Associated Press.
One survivor has been rescued by Japanese search teams but the six others remain missing.
The crash is the latest in recent series of accidents involving the USA military deployed to and near Japan.
Luka Modric wins Ballon d'Or 2018 ahead of Cristiano Ronaldo
Voting for the Ballon d'Or is conducted by an global panel of journalists who selected the victor from a list of 30 candidates. France worldwide and Paris Saint-Germain striker Kylian Mbappe has won the inaugural Kopa Trophy for the best Under-21 player.
The aircraft are a KC-130 refuelling plane and an F/A-18 Hornet fighter jet.
He says it is not clear what the weather conditions were like but overnight there was widespread cloud and rain across the Japanese archipelago.
Josef Patterson, a spokesman with the 3rd Marine Aircraft Wing, told Marine Corps Times.
In November previous year, a C-2 cargo plane carrying 11 passengers and supplies from the base at Iwakuni to the aircraft carrier USS Ronald Reagan crashed during an annual bilateral maritime field-training exercise with the MSDF.
The KC-130J conducts aerial refueling operations.
More than 50,000 USA troops are based in Japan under the bilateral security pact.