A search and rescue operation is under way in Japan after an incident involving two US Marine Corps aircraft.
The planes involved are a KC-130 and an F/A-18 based at Iwakuni near Hiroshima and were carrying seven personnel, Marines officials said.
One survivor has been rescued by Japanese search teams but the six others remain missing.
US media say the two planes crashed during mid-air refuelling but this has not been officially confirmed.
The Marine Corps tweeted that the incident occurred at about 02:00 on Thursday (17:00 GMT Wednesday).
There were five personnel on the C-130 and two on the F-18.
A Facebook posting by the III Marine Expeditionary Force said the incident took place 200 miles (320km) off the coast.
The US planes had taken off from Marine Corps Air Station Iwakuni and “were conducting regularly scheduled training when the mishap occurred”.
A Marines statement said: “We are thankful for the Japan Maritime Self-Defence Force’s efforts as they immediately responded in the search and rescue operation.”
The survivor is being assessed by doctors at MCAS Iwakuni.
The BBC’s Rupert Wingfield-Hayes in Tokyo says that air-to-air refuelling is a difficult and potentially dangerous flight operation, especially when done at night.
He says it is not clear what the weather conditions were like but overnight there was widespread cloud and rain across the Japanese archipelago.
The KC-130 is an extended-range tanker version of the C-130 and is used for mid-air refuelling.
The McDonnell Douglas F/A-18 Hornet is a fighter and attack aircraft and can carry a wide range of missiles and bombs.
The US has more than 50,000 troops stationed in Japan, more than 18,000 of them in the US Marine Corps.
The US has had problems with reliability of aircraft in Japan. In November an F/A-18 Hornet crashed into the sea south of Okinawa. The two pilots ejected and were rescued.
Last December part of a US helicopter crashed on to a school in Okinawa, renewing tensions with the local population.
Over the past years, a number of accidents and crimes have led to growing local opposition to the US base there.