Firefighters were hampered by erratic wind and low humidity Monday as they made slow progress against a fire that had burned nearly 12,000 acres and four houses in the Northern California wine country.

The fire, which started Friday northeast of here in Sonoma County, had been 20 percent contained, and full containment was not expected until Wednesday, said Janet Marshall, spokeswoman for the state Department of Forestry and Fire Protection.

The unpredictable wind and dry vegetation, compounded by uneven and often steep terrain, made the situation dangerous for the 1,875 firefighters, Marshall said.

"These are very erratic winds, which can be blowing perpendicular to one another," she said. "You never know where they're coming from, and that's a huge safety concern for firefighters."

A layer of moist air was expected in the area Tuesday morning, Marshall said. "If we get through the dry night, we can exhale tomorrow morning," she said.

Six firefighters were injured, none critically.

In addition to the four homes, eight outbuildings and 12 cars had been destroyed. About 40 residents had been evacuated from the area, about 60 miles north of San Francisco, including occupants of six houses ordered evacuated Monday morning, Marshall said.

Others living in the area were urged to have evacuation plans ready because the blaze was threatening 200 other houses and five businesses.

The fire also threatened major power lines from 21 generating plants in the Geysers, the world's largest geothermal power facility.

On Saturday, 14 of 21 generating plants and two transmission lines were idled, said Kent Robertson, a spokesman for Calpine Corp., which owns the lines. The company rerouted electricity from other plants, and no outages were reported.

In San Diego County, firefighters put out a brush fire that consumed 65 acres in the San Pasqual Valley. Several homes had been threatened, but no structures were damaged.

A crew keeps its eye on a backfire Sunday in Sonoma County, Calif.