The US Coast Guard has called off a search-and-rescue mission for nine missing airmen from Thursday's mid-air crash of a Coast Guard plane and Marine Corps helicopter off California's coast, a spokesman said.

"We no longer believe there is any chance anybody could be alive. We are moving into a salvage, recovery and investigative phase," said Coast Guard Rear Admiral Joseph Castillo.

Thursday's crash after sundown in an area set aside for military exercises by the Federal Aviation Administration, left two chopper pilots and a crew of seven aboard a Coast Guard C-130 transport plane missing in the Pacific Ocean.

The wreckage of the C-130 and the Cobra helicopter were found on Saturday, but no signs of crash victims. A Pentagon spokesman on Friday said it was "likely" the missing airmen had all perished.

The Coast Guard plane was in the area searching for a missing boater while the Marine helicopter was heading to San Clemente island, which is controlled by the navy, on a training mission.

Six Coast Guard cutters, three US Navy ships and various aircraft took part in the search and rescue mission.

The southern California Marine Air Wing suspended all flights through the weekend to re-emphasise safety measures after the crash.