TOKYO (AP) _ A U.S. Marine helicopter crashed upside down into the sea during night training exercises off Okinawa, and 14 Marines were missing today and feared dead, U.S. military spokesmen said.

Authorities said eight other Marines on board the CH-46 Sea Knight helicopter, based in Kaneohe Bay, Hawaii, were rescued shortly after the 11:55 p.m. crash Tuesday about 30 miles south of Okinawa's Kadena Air Force base.

The names of the missing were withheld pending notification of next of kin, the Navy said. The cause of the accident was under investigation.

The Sea Knight helicopter had just taken off from the USS Denver, said Marine Master Sgt. Jake Rodrigues, spokesman for U.S. Forces, Japan.

Rodrigues said the helicopter, built in 1967, was carrying a crew of four and 18 soldiers assigned to the 2nd Battalion, 3rd Marine Regiment, Okinawa, for six months' training as part of a unit-rotation program.

In Washington, Marine Corps spokesman Lt. Col. Fred Peck said the helicopter ''pitched over to the right and went into the water inverted, upside down. I have nothing to indicate why it did that.''

Peck said the four injured men were being treated aboard the Denver, an amphibious transport vessel designed for amphibious assaults and capable of carrying up to six CH-46 helicopters.

Capt. Lenny Ryan, a Kaneohe Marine Corps Air Station spokesman, said the four suffered minor injuries.

''An investigation will be initiated immediately,'' Ryan said. ''As soon as they're able to talk, we'll start getting information immediately so we can start trying to piece together what happened in this tragedy.''

He said the pilot was not wearing night-vision goggles at the time of the accident.

The helicopter went down 9 1/2 miles southeast of Kiyan Cape, said Nagatoshi Naruse, spokesman for the Maritime Safety Agency, Japan's coast guard. He said three Japanese aircraft and six Maritime Safety Agency patrol boats joined two U.S. vessels, including the Denver, and four U.S. aircraft in the search.

Another coast guard spokesman, Kazuhiro Sameshima, said the weather in the area was fair at the time of the crash, with waves about three feet high.

Okinawa, Japan's southernmost prefecture, has the highest concentration of Marines outside of the United States, the 20,000 Marines stationed there comprise about 10 percent of the corps.

Okinawa was under American administration from 1945 until it was turned over to the Japanese in 1972. Under a mutual security pact, 64,000 U.S. troops are stationed throughout Japan, including 35,000 on Okinawa.

The concentration of American troops on Okinawa, one of Japan's most densely populated prefectures, and the island's history as a bloody World War II battle site have made residents sensitive to training accidents.

Those sensitivities reached a boil this month when it was disclosed a hydrogen bomb fell off of a U.S. aircraft carrier in 1965 and remains lost at the bottom of the ocean only 80 miles from an inhabited Okinawan island.

The latest tragedy came after fires aboard three U.S. warships killed 55 sailors earlier this spring.

On April 19, an explosion and fire in a gun turret killed 47 sailors aboard the Norfolk-based battleship USS Iowa off Puerto Rico.

Six sailors died in an engine-room fire on the supply ship White Plains in the South China Sea on May 9.

An explosion and fire in a jet fuel pump room killed two sailors aboard the aircraft carrier America May 13 in the Atlantic Ocean.