The Reagan crew and Japanese forces were conducting search and rescue operations Wednesday afternoon and trying to recover the remaining crew and passengers.
"Personnel recovery is under way and their condition will be evaluated by USS Ronald Reagan medical staff", a US Navy statement said.
"From the United States military, we have received an initial report that engine trouble might be the cause", he said.
White House spokeswoman Lindsay Walters provided no additional details or comment on the incident.
It was flying to aircraft carrier USS Ronald Reagan in the Philippine Sea, when it crashed around 2:45 p.m. Japan time. "Prayers for all involved".
Japan's NHK news agency reports 8 people have been rescued so far.
A spokesman quoted Defense Minister Itsunori Onodera as telling reporters Wednesday that the C-2 aircraft crashed into the Pacific about 150 kilometers (90 miles) northwest of Okinotorishima, a Japanese atoll. The ship was taking part in Annual Exercise 2017 (AE17), a bilateral field-training exercise with the Japanese Navy conducted in waters off Japan from November 16 to November 26.
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