Four bodies have been recovered and up to 30 people are missing after a pre-dawn fire sank a scuba diving boat off a Southern California island, the US Coast Guard says.

A distress call was reported at 3.15am local time as the vessel burned near Santa Cruz Island, the Coast Guard said in a statement.

Five crew members of the diving boat Conception were rescued by a “good Samaritan pleasure craft”.

“The crew was already awake and on the bridge and jumped off,” Coast Guard Captain Monica Rochester said.

The passengers were all asleep below decks, according to information the Coast Guard received.

James Kohl, brother of one of the missing passengers, told KTLA TV in Los Angeles he was told there was a propane gas explosion on board, but Ms Rochester said she could not confirm that.

US Coast Guard Lt. Commander Matthew Kroll told CNN the cause of the fire was still undetermined. “It happened quickly enough so many people could not get off,” he added.

The boat sank in 20m of water not far from the Santa Cruz shore, with only the tip of its bow showing.

The Conception was used for excursions to the Channel Islands.

It was on the final day of a three-day scuba diving trip for the Labor Day weekend.

– Reuters / BBC