The death toll in a fire that devastated an Oakland, California, loft building during a weekend dance party has climbed to 30 people and is expected to rise further as recovery teams push deeper into the charred ruin, authorities said on Sunday.

The remains of the 30 victims were found as authorities sifted through the debris-filled shell of the two-story converted warehouse being used by an artists' collective.

"That is an astronomical number," Sergeant Ray Kelly, a spokesman for the Alameda County Sheriff's Office, told a news briefing. "We're still not done."

Kelly warned earlier that the number of fatalities would likely rise in what fire officials called the deadliest blaze in the Bay Area city's history. He said names of the victims would be released "in the coming hours" after their families were notified.

"In regards to the amount of people that are still missing, yes, it's a significant number, Kelly said.

The fire erupted about 11:30 p.m. on Friday (0730 GMT on Saturday) during the party featuring electronic dance music that dozens of people attended.

Only 20 percent of the building had been searched in the past 12 hours, Melinda Drayton, battalion chief at the Oakland Fire Department, said on Sunday morning. She said the origin of the fire had yet to be determined and the cause was unknown.

The recovery operation had been delayed for hours until workers could enter safely. The roof had collapsed onto the second floor and in some spots, the second story had fallen onto the first.