Nine US military and Coast Guard personnel are feared dead after a mid-air collision between a transport plane and helicopter over the Pacific Ocean, authorities said.
As dawn broke early on Friday, an extensive search was under way for survivors of the crash near debris spotted in waters off the southern California city of San Diego, authorities said.
It followed the collision late on Thursday between a Coast Guard transport plane and a United States Marine Corps light attack helicopter.
"The search is still on but it's likely taken the lives of nine individuals," Pentagon spokesman Bryan Whitman told reporters, describing the crash as a "tragic event."
Three Coast Guard cutters and a helicopter were scouring the area for survivors along with four US Navy vessels and several military helicopters.
Seven people were aboard the Coast Guard C-130, and two were in the Cobra attack helicopter, officials said.
Aerial and maritime search efforts were focused on 1,046 square kilometres of ocean, officials said at a noon briefing.
US Coast Guard Rear Admiral Joseph Castillo refused to give up hope of finding survivors, saying teams were in a "search-and-rescue phase."
"Our people are very highly trained in survival techniques," Castillo said. "They are very highly fit, physically fit. They are able to survive things that you would ... be surprised at."
Castillo would not speculate on the possible cause of the crash.
"We're proceeding down the path of the investigations that are necessary and appropriate to determine what exactly happened and what we can do to prevent things like this from happening," Castillo said.
"Those investigations will be done very closely in lock-step between the Marine Corps and the Coast Guard."
The Marine Corps aircraft, based at the sprawling Camp Pendleton base outside San Diego, was on a training exercise when the accident occurred.
It is the third accident involving a military helicopter in the region this year. On May 5, two Marines died when their Super Cobra helicopter crashed during a training flight while five crew died when a Navy HH-60H Seahawk helicopter crashed into the Pacific Ocean on May 19.
