Two U.S. Marine Corps helicopters, each carrying six crew members, collided and went down off the Hawaiian island of Oahu while on a nighttime training mission, military officials said Friday, and rescuers are searching choppy waters where debris had been sighted.

There was no immediate word on the fate of those aboard or what caused the accident.

The transport helicopters known as CH-53Es crashed just before midnight Thursday, officials said.

A Coast Guard helicopter and C-130 airplane spotted a debris field four kilometres offshore early Friday. The debris covered an area of three kilometres, Marine Capt. Timothy Irish said.

The choppers are part of the 1st Marine Aircraft Wing. Known as Super Stallions, they are the U.S. military's largest helicopter, capable of carrying a light armoured vehicle, 16 tons of cargo or a team of combat-equipped Marines, according to a Marine Corps website.

Hawaii Helicopter Collison

U.S. Marines walk on the beach at Waimea Bay, near Haleiwa, Hawaii, where two military helicopters carrying six crew members apiece crashed offshore. (Mariana Keller/Associated Press)

Elaray Navarro, a retiree who lives across the street from the beach, said she heard two booms late Thursday that were loud enough to shake her house.

"I threw my blanket off, put my slippers on and ran outside thinking it was a car accident," she said.

She expressed concern for the crew as she watched the pounding surf from a beach in Haleiwa. "I pray to the man upstairs to help them. To bring them home safely," she said.

The search included aircraft from the navy and air force, a Honolulu Fire Department rescue boat and Coast Guard cutters, officials said.

About two dozen Marines gathered Friday morning on the beach at Waimea Bay, a popular surfing spot a few kilometres from the rescue operation.

A steady rain was falling on the North Shore just before daybreak, and rough water conditions were forecast.

A swell approaching the area was to bring dangerous 9- to 12-metre waves to beaches and 3- to 6-metre seas near the rescue operation, National Weather Service meteorologist Matthew Foster said. Winds were expected to be relatively calm at 16 km/h or less.

The crash comes less than a year after the Marine Corps' hybridized tilt-rotor aircraft crashed in during a training exercise, killing two Marines. The MV-22 Osprey went down last May with 21 Marines and a Navy corpsman on board.

In 2011, one serviceman was killed and three others were injured when a CH-53D Sea Stallion chopper crashed in Kaneohe Bay, Hawaii.

Haleiwa, Hawaii map

A debris field was spotted Friday about four kilometres off the coast of Oahu, near the town of Haleiwa, after the collision between two Marine Corps helicopters was reported. (Google Maps)

With files from Reuters