A search is underway for nine people missing after a US Coast Guard plane collided with a Marine helicopter over the Pacific Ocean near Southern California. Aviation officials said the Coast Guard C-130 plane carrying seven people collided with the Marine Corps AH-1 Super Cobra attack helicopter carrying two people. The collision is believed to have occured about 27 kilometrs east of the US Navy-owned San Clemente Island. The officials said a large rescue team from the Navy and Coast Guard were combing the area.