Cal Fire spokeswoman Julie Hutchinson said the residents of those homes were under mandatory evacuation, the San Francisco Chronicle reported.
"They are making some progress, but it is burning some pretty thick heavy brush and now threatening some homes," she said.
"It's a steep and rugged area there, steep terrain with poor access."
About 175 firefighters were working to put out the blaze.
The cause of the fire, first reported at 1:15 p.m. PDT, had not been determined, the newspaper said.