Authorities have said they fear the death toll could climb to 40 as they sift through the rubble of the converted warehouse.
"We have confirmed that the count of the deceased at 30. That is an astronomical number," Sergeant Ray Kelly of the Alameda County Sheriff's Department told reporters.
Authorities had only searched a small part of the two-story building destroyed by the inferno late Friday night.
"We are still not done... they are working and pulling this building apart and dissecting it. And so we're really starting to get deeper into the building. As we do that, we continue to find more victims," Kelly said.
Authorities have said they fear the death toll could climb to 40 as they sift through the rubble of the converted warehouse in the city of Oakland, located near San Francisco.
Kelly earlier said that only about 20 percent of the building had been searched.
Artists and students had been living and working in the cluttered warehouse, even though the site was not licensed for such use.
The electronic dance music party, which had between 50 and 100 guests, also took place without proper permits.
The cause of the blaze was still under investigation.
Authorities on Sunday asked relatives of the missing to "eliminate future delays" in identifying the victims and preserve items like hairbrushes and toothbrushes for DNA samples.
Alameda County Sheriff's Office spokesman Sgt. Ray Kelly addresses the media December 4, 2016 in Oakland, California
"We will ask for them as we need them," said Captain Melanie Ditzenberger of the sheriff's department coroner's bureau.
Kelly said only three victims' families have received confirmed death notices because of difficulties identifying the charred remains.
Firefighters worked for 12 hours by night to enter the building, eventually breaching a wall to clear their way into the warehouse, said Oakland Fire Battalion Chief Melinda Drayton.
"It will take a few more days just getting through the building," Drayton told reporters.
A warehouse smoulders after it was destroyed by a fire, December 3, 2016 in Oakland, California warehouse fire
"It was filled end-to-end with furniture, whatnot, collections," Fire Chief Teresa Deloach Reed said Saturday. "It was like a maze, almost."
"There wasn't a real entry or exit path," she said.
In addition, firefighters said the building seemed to have no sprinklers or smoke detectors.
Officials said the roof collapsed onto the second floor, which was connected to the ground floor only by a makeshift system of wooden pallets.
A dozen bodies were found in an area in the middle of the building, Drayton said.
She added that the firefighters had taken every precaution to treat the human remains with respect.
Describing the atmosphere during the night of excavation, Drayton choked back tears: "It was quiet. It was heartbreaking."
Most of those who perished in the blaze, which started about 11:30 pm Friday (0730 GMT Saturday) were thought to have died on the upper floor of the warehouse known as the Oakland Ghost Ship, Reed said.
Some of the missing were apparently foreign students thought to be in their 20s and 30s, making their identification more difficult.
The fire was the deadliest incident in Oakland since the 1989 Loma Prieta earthquake in northern California, which killed 63 people.
The deadliest nightclub fire in the United States in recent decades occurred in 2003, when pyrotechnic effects by the rock band Great White set off an inferno at The Station nightclub in Rhode Island, killing 100 people.
Oakland, a city of some 420,000, was once deemed largely unsafe but is now home to a more affluent population attracted by affordable rent and proximity to San Francisco.