Four bodies were recovered by rescuers searching for 34 people missing after a pre-dawn fire sank a scuba diving boat off a southern California island, a US Coast Guard spokesman said.

No other information was immediately available about the genders, identities or the conditions of the recovered bodies, Coast Guard Petty Officer Mark Barney said by phone.

The search for the missing was still under way, he said.

Five crew members were rescued earlier by a "good Samaritan pleasure craft" after a distress call was reported at 3:15am local time as the 23 metre (75ft) vessel burned about 18 metres (20 yards) off of Santa Cruz Island, the Coast Guard said in a statement.

"The crew was already awake and on the bridge and jumped off," Coast Guard Captain Monica Rochester told a news conference at Oxnard. The passengers were all asleep below decks, according to information the Coast Guard received, Captain Rochester said.

US Representative Julia Brownley, a Democrat whose district includes Ventura County, told reporters in Oxnard: "They have determined that there are some bodies there. Devastating and tragic, and we still are hoping that there could still be some survivors."

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 Captain Rochester said she could not confirm that.

Asked about 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.

Local media reported foggy conditions along the southern California coast.

Of the five persons rescued near the coast of Santa Barbara, at least one had minor injuries, the Coast Guard said.