SAN FRANCISCO (AP) — Fire officials say a blaze in Northern California that drove more than 1,000 people to flee their homes grew overnight and was heading toward a sparsely populated area.

California's Department of Forestry and Fire Protection spokeswoman Emily Smith said Tuesday that the fire in Lake County north of San Francisco is now nearly 18 square miles (46 square kilometers).

The blaze burning through dry brush, grass and timber has destroyed 12 homes since it started on Saturday and is threatening 600 structures.

About 1,500 people remain under mandatory evacuation orders.

Officials say unusually hot weather, high winds and highly flammable vegetation turned brittle by drought helped fuel several fires that began over the weekend.

Those same conditions led to the state's deadliest and most destructive fire year in 2017.