The US Navy has said an aircraft carrying 11 crew and passengers has crashed into the Pacific Ocean.
The 7th fleet, based in Japan, said a search and rescue operation was under way.
Japan says a search and rescue operation has found eight of those who were on board the C-2 "Greyhound" transport aircraft - but it is not clear if they are alive.
The names those on board are being withheld until next of kin can be informed.
USS Ronald Reagan
The aircraft had been en route toward the aircraft carrier USS Ronald Reagan and the cause of the crash was unclear, the US Navy said in a statement.
"A United States Navy aircraft carrying 11 crew and passengers crashed into the ocean southeast of Okinawa," it said.
"Personnel recovery is under way and their condition will be evaluated by USS Ronald Reagan medical staff."
The USS Ronald Reagan is now conducting the search and rescue operations.
In Tokyo, Defence Minister Itsunori Onodera said a joint operation had been launched.
"From the US military, we have received an initial report that engine trouble might be the cause," he said.
The aircraft was operating in the Philippine Sea when the crash happened at 2.45pm local time (approximately 5.45am GMT).
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 others.