Washington (AFP) - The US Coast Guard was searching for two military helicopters that reportedly crashed off the Hawaiian island of Oahu while carrying a total of 12 people, officials said Friday.

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 have found none of the crew members, she said.

The Marine Corps said there was an "ongoing search and rescue" operation for the two CH-53 helicopters. The aircraft had been on a training mission.

"Our heartfelt condolences go out to the families of all those involved in the mishap, and (we) express our deepest gratitude for the heroic, selfless actions of the civilian and military first responders," Marine Major Christian Devine said.

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.

A Navy helicopter and a chopper from the Honolulu fire department joined the search, as did 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 aiding with earthquake relief in Nepal crashed in bad weather, killing six Marines and seven other people.

And in March, 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