Up to 40 people were feared dead in a huge fire that tore through a rave party in a warehouse near San Francisco, as authorities warned Saturday of a prolonged search and recovery effort.

The building in Oakland lacked permits for people to live and work as a group of artists did, and for the party to take place, officials said, adding that it also had no smoke detectors or sprinklers.

The interim chief of Oakland's Planning Department, Darin Ranelletti, said the city had recently received complaints against the building for debris and trash placed in adjacent vacant lots, and illegal construction inside the building.

Firefighters at the scene had to pull out of the building to shore it up when part of the fragile structure and some of the walls began to move. The roof eventually collapsed.

Nine bodies have been recovered so far.

Sergeant Ray Kelly of the Alameda County Sheriff's department told an evening news conference that about two dozen people who were reported missing had been located.

But at least two dozen more remain missing, he said.

"We don't know how far into the process we are, because we don't absolutely have a number of people that we know are deceased inside of there," he said.

Most of those who perished in the blaze that started about 11:30 pm (0730 GMT) Friday were thought to have died on the upper floor of the two-story warehouse known as Oakland Ghost Ship, Oakland Fire Chief Teresa Deloach Reed said.

"It must have been a very fast-moving fire," she said.

It was not immediately known what started the inferno at the electronic dance music party attended by an estimated 50 to 100 people.

- 'Expecting the worst' -
Some of the missing are from overseas, making identification of the victims -- thought to be in their 20s and 30s -- more difficult.

The warehouse had numerous partitions that had been added and a makeshift stairwell built from pallets.

Some of the structural changes made it extremely difficult for people to escape, Reed said.

"There wasn't a real entry or exit path," Reed said.

The clutter hampered firefighters' efforts to put out the blaze.

"It was filled end-to-end with furniture, whatnot, collections," Reed said. "It was like a maze almost."

It appeared no smoke detectors were activated in the building, which also had no sprinkler system, she added.

Friends and families of partygoers took to social media to search for news about their loved ones, with some posting information on the event's Facebook page.

"Please tell me you are safe," one woman wrote, adding a friend's name, while others posted prayers.

- 'Felt my skin peeling'-
"I literally felt my skin peeling and my lungs being suffocated by smoke," Bob Mule, a photographer who lives in the building, told Fox television affiliate KTVU. "I couldn't get the fire extinguisher to work."

Another artist told the station that the fire broke out in the back of the building where some 18 artists shared space.

The man, who was not identified, said he had tried to help a fellow artist who had broken his ankle flee the inferno, but was obstructed by the smoke and flames as well as the mess of objects.

"I hope he is ok," he told the station, his voice breaking up.

During the blaze, orange flames shot into the sky from the roof and flared out of the large windows.

The fire was described as 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 broke out in 2003, when pyrotechnic effects by the rock band Great White set off an inferno at The Station nightclub in Rhode Island, killing about 100 people.