The 75-foot boat caught fire near Santa Cruz Island off the coast from Los Angeles (AFP)

Eight people were dead and more than two dozen missing and feared deceased Monday after a scuba diving boat caught fire and sank off the California coast, with passengers trapped below deck by the roaring blaze.

Fire crews in helicopters, small boats and a Coast Guard cutter battled the fierce pre-dawn fire on the 75-foot (23-meter) Conception, which had been on a diving excursion around Santa Cruz Island, just west of Santa Barbara in southern California.

But the blaze and intense heat prevented them from breaching the vessel's hull to search for survivors before the craft sank, the Coast Guard said. A dense fog further complicated rescue efforts.

"Four victims have been recovered thus far as deceased," Santa Barbara County Sheriff Bill Brown told a news conference.

"Rescue and recovery efforts on the scene have located an additional four victims on the ocean floor in close proximity to the vessel," Brown said, while 26 people are missing.

Five Conception crew members were awake and jumped into the water when flames burst out around 3:15 am (1015 GMT), Coast Guard Captain Monica Rochester said, putting the total number of people aboard the boat at 39.

The five were rescued by people on a pleasure craft called the Grape Escape, Rochester said.

Shirley Hansen, who was on the Grape Escape with her husband Bob, told the Los Angeles Times they were asleep when they heard pounding on the side of their fishing boat.

The crew, some only in underwear and two with leg injuries, had retrieved a dinghy and paddled 200 yards to the Hansens' boat.

Shirley Hansen said the men were distraught -- one had a girlfriend below-deck on the Conception -- and two of the men paddled back to look for survivors, but found none.

"Engulfed in flames"

Rochester of the Coast Guard said all the passengers were believed to have been sleeping at the time. The Conception had a crowded cabin with three-high bunks below decks.

The Conception sank 20 yards (meters) off the island's northern shore, leaving only its bow exposed.

US news outlets released audio of a distress call in which a crew member on the boat yells, "Mayday! Mayday! Mayday!" and "I can't breathe!"

A Coast Guard operator asks the man if the passengers can get off the boat and if the crew has fire extinguishers, but the response is inaudible.

Rochester said the Conception, which was launched in 1981 by a Santa Barbara-based company called Truth Aquatics, "has been in full compliance" with safety regulations, and that its owner was cooperating with investigators.

Glen Fritzler, listed as owner on the Truth Aquatics website, was a respected professional who only months ago received the California Scuba Service Award, according to the Los Angeles Times.

Asked whether there had been an explosion on board or a slow-developing fire, Rochester said that "the only Mayday call we received" -- which came from the boat -- "was the vessel was engulfed in flames."

The fire was put out multiple times but flared back up, the Coast Guard said, apparently because of the amount of fuel in the vessel. The boat could carry up to 1,600 gallons (6,057 liters), according to the company website.

The Truth Aquatics website said the Conception, listed as having bunks for up to 46 people, had been scheduled to return Monday from a three-day trip after visiting several diving spots around Santa Cruz Island.

The area is popular for a variety of water and outdoor sports.