The US Coast Guard is searching for two military helicopters that have crashed over Hawaii while carrying 12 people.

Coast Guard spokesperson Sara Mooers from Honolulu said they were notified on Thursday night of a collision between two Marine Corps aircraft carrying six crew members each.

Search aircraft have spotted a debris field and an empty life raft about 4kms off the north shore of the Hawaiian island of Oahu, but so far no trace of the crew.

It is not yet clear what caused the collision.

Major Christian Devine said: "The Marine Corps can confirm there's an active search and rescue operation ongoing for two CH-53 helicopters off the coast of Oahu".

"The US Coast Guard is currently conducting search and rescue operations".

He added that the aircraft were from the nearby Kaneohe Bay US Marine Corps Air Station.

The search may be hampered by a high surf warning issued for Oahu by forecasters.

The US National Weather Service predicted waves of 30-40ft on Friday morning.