The US Coastguard said one of its planes collided with a military helicopter off the Southern California coast early today.
According to the coastguard, the US Navy reported the crash at about 7pm local time yesterday. The helicopter went down about 15 miles east of San Clemente Island, the furthest south of the Channel Islands.
Coastguard Petty Officer Allyson Conroy said several vessels were scouring the area.
She did not immediately say how many people were on the helicopter or release any other details on the crash.
A pilot reported seeing a fireball near the suspected crash site, Federal Aviation Administration spokesman Ian Gregor said.
Mr Gregor said preliminary information indicated the crash involved a Marine Corps AH-1 Cobra and a coastguard C-130 transport plane.
There were seven people on board the C-130 and two on the chopper, he said.
Marine Corps spokesman Cpl Michael Stevens said an AH-1 Cobra had crashed. He had no additional details.
San Clemente Island is 68 miles west of San Diego.