TOKYO (Reuters) – A U.S. Navy transport plane carrying 11 people crashed in the Philippines Sea south of Japan on Wednesday as it flew to the aircraft carrier the USS Ronald Reagan, the U.S. Seventh Fleet said.

Eight people had been rescued, with the remaining three unaccounted for, Japan’s public broadcaster NHK reported. A spokesman for the U.S. Navy in Japan said that it had no information on whether anyone on board had been picked up.

“USS Ronald Reagan is conducting search and rescue operations. The cause of the crash is not yet known,” it said in a press release.

Japanese Minister of Defence Itsunori Onodera told reporters the U.S. Navy had informed him that the crash in the Philippine Sea may have been a result of engine trouble.

The propeller powered transport plane, a C-2 Greyhound, carries personnel, mail and other cargo from mainland bases to carriers operating at sea.

C-2 planes have been in operation for more than five decades and are due to be replaced by the long-range tilt-rotor Osprey aircraft.