The C2-A Greyhound aircraft was on a routine flight from Marine Corps Air Station Iwakuni in southern Japan to the USS Ronald Reagan aircraft carrier, which is now in the Philippine Sea on exercises with Japan's Maritime Self-Defense Force.
A U.S. Navy airplane has crashed in the Philippine Sea near the coast of Okinawa, Japan, the Pacific 7th Fleet command said in a statement on Wednesday.
It is thought at least three people are missing.
The crash, which took place about 500 nautical miles (575 miles) southeast of Okinawa, Japan, happened at 2:45 p.m. local time, which is 12:45 a.m. ET. The cause of the crash was not immediately known and an investigation would be carried out, it said.
Japanese and American rescuers scrambled to reach the downed aircraft in seas off the remote Japanese reef of Okinotori, in the latest accident to hit American armed forces in East Asia.
"From the USA military, we have received an initial report that engine trouble might be the cause", he said.
They were transferred to the USS Ronald Reagan for medical evaluation. "Prayers for all involved".
White House spokeswoman Lindsay Walters provided no additional details or comment on the incident.
The Reagan crew and Japanese forces were conducting search and rescue operations Wednesday afternoon and trying to recover the remaining crew and passengers.
It was reported that the plane crashed while it was on its way to the USS Ronald Reagan aircraft carrier, officials confirm.
Two months earlier in June, the USS Fitzgerald and a cargo ship smashed into each other off Japan, leaving seven sailors dead.
The Navy has concluded that the collisions were avoidable and resulted from widespread failures by the crews and commanders, who didn't quickly recognize and respond to unfolding emergencies.