Four firefighters were injured on Saturday in a fast-moving wildfire in Northern California that forced the evacuation of thousands of people, officials said.

The firefighters suffered second-degree burns as they battled the so-called Valley Fire, in Lake County, north of San Francisco, said Cal Fire spokesman Daniel Berlant.

The Valley Fire broke out on Saturday afternoon and quickly spread to 10,000 acres (4,047 hectares), the California Department of Forestry and Fire Protection (Cal Fire) said on its website.

The firefighters had been dropped off by helicopter to build containment lines. They were airlifted to UC Davis Medical Center in Sacramento where they were listed in stable condition, Berlant later said on Twitter.

The explosive growth of the Valley Fire forced authorities to order mandatory evacuations of a number of communities, including all of Hidden Valley Lake which has about 5,500 people, according to a Lake County Sheriff's Office statement.

About 100 miles (161 km) to the southeast, the so-called Butte Fire has destroyed 86 homes and 51 outbuildings in rural Amador and Calaveras counties, where it covers an estimated 65,000 acres (26,305 hectares), officials said. It is only 15 percent contained.

Thousands of residents in the area were required to evacuate on Friday, and the blaze threatened more than 6,000 structures, officials said.