Image copyright AP Image caption Search teams combed nearby beaches looking for debris

The US Coast Guard is halting its search for 12 marines missing since two helicopters crashed near the Hawaiian island of Oahu.

The incident happened late on Thursday last week. Hours later debris was seen floating in the ocean.

Round-the-clock efforts were ended after more than 40,000 sq mi had been searched, a Coast Guard statement said.

Captain Jim Jenkins said it had been an "extremely difficult" decision to halt the search.

"I know I speak for the entire Coast Guard when I say our thoughts and prayers are with Marine Corps helicopter squadron and particularly with families and loved ones of those missing," the commander of the 14th District said.

His branch of the coast guard was ready to assist the marines in any further searches, he said.

It remains unknown why the two helicopters crashed. Initial reports said they had collided, but the marines have since said this is unconfirmed.

All four life rafts from the helicopters have been recovered with no sign of survivors, according to Hawaii News Now.