There was no immediate word on what caused the crash or if any survivors had been found.
The transport helicopters each had a crew of six from Marine Corps Base Hawaii and crashed just before midnight Thursday, officials said.
The CH-53Es came from the 1st Marine Aircraft Wing, Marine Capt. Timothy Irish said. Known as Super Stallions, they are the U.S. military's largest helicopter, capable of carrying a light armored vehicle, 16 tons of cargo or a team of combat-equipped Marines, according to a Marine Corps website.
A Coast Guard helicopter and C-130 airplane spotted a debris field offshore early Friday.
A swell approaching the area will bring dangerously high waves to beaches and seas of up to 20 feet (6 meters) near the rescue operation, National Weather Service meteorologist Matthew Foster said. Winds will be relatively calm.

