Six US marines were missing after two American military aircraft crashed on Thursday during a refuelling operation off the coast of Japan, a US defense official told media.
According to the statement, the two planes -- an F-18 fighter and a C-130 tanker – took off about 200 miles from the coast of Japan around 2:00 am.
Search and rescue operations were underway following a "mishap." The planes "were conducting regularly scheduled training when the mishap occurred," the statement said.
US media also reported five crew on the C-130 and two on the F-18.
"We are thankful for the Japan Maritime Self-Defence Force's efforts as they immediately responded in the search and rescue operation," the Marines said.
The planes took off from Marine Corps Air Station Iwakuni in southern Japan and were "conducting regularly scheduled training when the mishap occurred," the statement said.
The circumstances of the mishap are currently under investigation, the military added.
The US military has about 50,000 troops stationed in Japan and accidents are not uncommon.
In November, a US navy fighter jet crashed into the sea off Japan's southern island of Okinawa and its two crew members were rescued alive.
The US military has also experienced difficulties with its Osprey helicopters, with several emergency landings, a deadly crash and a piece of the chopper falling on the grounds of a Japanese school.
Those incidents have stoked tensions between close military allies Washington and Tokyo and led to protests against the deployment of Ospreys by residents living near US bases.