Two US Marine helicopters have collided near the Hawaiian islandof Oahu, each with six people on board.
The crash occurred during a night-time training mission late on Thursday but it is unclear why.
Coast Guard Chief Petty Officer Sara Mooers said debris was seen in the ocean.
Marine Capt Timothy Irish told the Associated Press news agency that a search and rescue was under way for the two CH-53 transport helicopters.
The aircraft were from the 1st Marine Aircraft Wing from Marine Corps Base Hawaii, he said.
Ms Mooers told Los Angeles radio station KNX-AM the search would be tough because of darkness and a high surf advisory.
Less than a year ago, a Marine Corps MV-22 Osprey crashed during a training exercise, killing two Marines.