Nine US military and Coast Guard personnel were missing on Friday 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.

"We are always hopeful ... the assumption is always that they are alive," said US Coast Guard Captain Thomas Farris, noting a person could survive in the water for 19-20 hours, possibly longer, depending on body type and clothing.

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 believed to be aboard the Coast Guard C-130, and two were in the Cobra attack helicopter, officials said.

The cause of the crash was not immediately known.

Farris said the Coast Guard plane had been searching for a missing person at the time of the accident. The subject of the initial search remains missing.

The missing 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 5 May, 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 19 May.

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AFP

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