The US Navy has said a C-2 'Greyhound' aircraft carrying 11 crew and passengers has crashed into the Pacific Ocean.
A search and rescue operation has found eight of those who were on board, the 7th fleet said.
But the condition of the eight found is not known.
"A United States Navy aircraft carrying 11 crew and passengers crashed into the ocean southeast of Okinawa," a statement from the Navy said.
"Personnel recovery is under way and their condition will be evaluated by USS Ronald Reagan medical staff.
"The aircraft was en route to the US Navy aircraft carrier USS Ronald Reagan, which is currently operating in the Philippine Sea."USS Ronald Reagan is conducting search and rescue operations. The cause of the crash is not known at this time."
In Tokyo, Defence Minister Itsunori Onodera said a joint operation had been launched.
"Currently a search-and-rescue mission is being conducted in the area by the US side as well as the Japan Maritime Self Defence Force," he said.
"From the US military, we have received an initial report that engine trouble might be the cause."
It was operating in the Philippine Sea when the crash happened at 2.45pm local time (approximately 5.45am GMT).
Names are being withheld before next of kin can be informed.
The US Navy 7th fleet has 60-70 ships, 200-300 aircraft and 40,000 Navy and Marine Corps personnel assigned to it at any given time, according to its Twitter page.
The C-2 Greyhound aircraft is a twin-engine, high-wing cargo aircraft, with the primary job of carrying supplies and passengers to and from the US aircraft carriers.
In August, the USS John S McCain crashed with a tanker off Singapore, killing 10 sailors and injuring five more.
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