Hundreds of firefighters are battling a fast-growing wildfire that is threatening homes and wineries in California's Napa Valley.

Fire officials say the blaze has burned 300 acres in the rugged hills just outside the wine country town of St. Helena. The fire was about 40 percent contained Saturday morning.

CalFire spokeswoman Nancy Carniglia says the fire has destroyed at least two structures and threatens 200 homes and several wineries.

About 100 residents left their homes under voluntary evacuation orders Friday night, and an evacuation center has been set up at St. Helena High School.

The cause of the fire is still being investigated.