Nine victims of a mid-air collision between a coastguard aircraft and a US Marine Corps helicopter were feared dead today as the investigation focused on how the crews failed to see each other in a heavily-used military training area.

Military aircraft and ships searched the ocean off Southern California for any sign of the victims yesterday, while investigators gathered recordings of air traffic controllers and pilot communications.

The search covered 644 square miles of ocean but focused on a debris field 50 miles off the coast of San Diego, California.

The crash involved a coastguard C-130 plane with a seven-member crew and a Marine Corps AH-1W Super Cobra helicopter with two aboard as it flew in formation near the US Navy’s San Clemente Island, a site with training ranges for amphibious, air, surface and undersea warfare.

It was not known whether the pilots were aware of each other before the collision, which happened at 7.10pm local time on Thursday.

Pentagon spokesman Bryan Whitman described the crash as “a tragic event” and said: “The search is still on, but it’s likely taken the lives of nine individuals.”

In San Diego, crews of navy ships, coastguard cutters and helicopters planned to keep scouring the ocean even though nearly a full day had passed since the accident.

“We’re still in the search-and-rescue phase, we are not standing down from that at this point,” said Captain Tom Farris, commander of the US Coastguard’s San Diego sector. “We have every hope we will find survivors.”

The identities of the crew members were not immediately known. The C-130 crew had survival gear aboard the aircraft, including exposure suits that could have allowed them to survive in the water for hours, Petty Officer Henry Dunphy said.

The Sacramento-based C-130 crew was looking for a man on 12ft motorised skiff who was reported missing after leaving Avalon Harbour on Santa Catalina Island to reach a friend on a disabled yacht adrift off Catalina in high winds, authorities said.

The Marine Corps helicopter was flying from Camp Pendleton near San Diego to San Clemente Island, said Major Jay Delarosa, a spokesman for Marine Corps Air Station Miramar in San Diego.

Two Super Cobras, a type of attack helicopter, were escorting two big CH-53E Super Stallion transport helicopters carrying Marines to the island, Maj Delarosa said. He did not know how many Marines were aboard the transports.

After the collision, the other three helicopters returned to base, he said.

The accident occurred in airspace uncontrolled by the FAA and inside a so-called military warning area, which is at times open to civilian aircraft and at times closed for military use, said Federal Aviation Administration spokesman Ian Gregor.

He said he did not know the status of the airspace at the time.

Minutes before the collision, the FAA told the C-130 pilot to begin communicating with military controllers at Naval Air Station North Island in San Diego Bay, but it was not known if the pilot did so, Mr Gregor said.

He said FAA controllers never communicated with the Cobra pilots.

Capt Farris said it was not unusual to have a high volume of military traffic working in training areas.

He said pilots in the area were responsible for seeing other’s aircraft around them.