The C-2 "Greyhound" transport aircraft came down about 500 nautical miles (925 kilometers) southeast of Okinawa as it was bringing passengers and cargo from Japan to the USS Ronald Reagan aircraft carrier, the Navy said in a statement.
The Reagan was operating in the Philippine Sea during a joint exercise with Japan's Maritime Self-Defense Force when the twin-propeller plane crashed at 2:45 p.m. Japan time. The cause of the crash was not immediately clear and the incident will be investigated, the Navy said.
Eight people were rescued about 40 minutes later. They were taken to the Reagan for medical evaluation and are in good condition, the Navy said.
U.S. and Japanese naval ships and aircraft are searching for the missing. Japan's Defense Ministry said the crash site is about 150 kilometers (90 miles) northwest of Okinotorishima, a Japanese atoll.
The names of the crew and passengers are being withheld pending notification of next of kin.
Philippine military spokesman Maj. Gen. Restituto Padilla said his military alerted its ships and aircraft shortly after learning about the crash but could not provide help because of the distance from the country.
The Nov. 16-26 joint exercise in waters off Okinawa has been described by the Navy as the "premier training event" between the U.S. and Japanese navies, designed to increase defensive readiness and interoperability in air and sea operations.
The Navy's Japan-based 7th Fleet has had two fatal accidents in Asian waters this year, leaving 17 sailors dead and prompting the removal of eight top Navy officers from their posts, including the 7th Fleet commander.
The USS John S. McCain and an oil tanker collided near Singapore in August, leaving 10 U.S. sailors dead. Seven sailors died in June when the USS Fitzgerald and a container ship collided off Japan.
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. A Navy report recommended numerous changes to address the problems, ranging from improved training to increasing sleep and stress management for sailors.
Associated Press writers Yuri Kageyama in Tokyo and Jim Gomez in Manila, Philippines, contributed to this story.
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