The San Jose Mercury News quoted fire officials as saying they were told up to 70 people were at the Golden Donna's 100% Silk West Coast tour event.
The fire broke out around 11:30 PM (1300 IST) and was brought under control by early morning, with crews sifting through the rubble searching for victims, fire officials said.
"We still have to do a more thorough search of the building and we don't know the potential number of other victims," the East Bay Times newspaper quoted Fire Chief Teresa Deloach-Reed as saying.
Friends and family of partygoers went to social media to try and find news about their loved ones, with some posting information on the event's Facebook page.