The Coast Guard was alerted late Thursday to the reported collision between two Marine Corps choppers carrying six crew members each, said Coast Guard spokeswoman Sara Mooers from Honolulu.
Coast Guard search aircraft have spotted a debris field about 2.5 miles (four kilometers) off the north shore of Oahu but no people, she said.
The Marine Corps said on Twitter there was an "active search and rescue" for two CH-53 helicopters off the coast of Oahu.
The aircraft involved in the accident were attached to Marine Heavy Helicopter Squadron 463 of the service's 1st Marine Air Wing in Hawaii.
The Coast Guard sent a helicopter and an HC-130 Hercules plane to take part in the search.
Also searching are a Navy helicopter and one from the Honolulu fire department, as well as a rescue boat, Mooers said.
US Marines have been involved in several helicopter mishaps in recent months.
In September, one Marine was killed and 11 others injured when their CH-53E made a hard landing while training at the Camp Lejeune military base in North Carolina.
In May, a Marine helicopter conducting earthquake relief in Nepal crashed in bad weather, killing six Marines and seven other people.
And in March 2015, 11 Marines and National Guardsmen died when their Black Hawk helicopter crashed off Florida's coast, an accident blamed on pilot error in bad weather.
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