Thousands of people rushed to escape a massive wildfire charging across California's tinder-dry Sierra Nevada foothills and another out-of-control fire that broke out in Northern California, injuring four firefighters.

The northern fire began in Lake County, 100 miles (160km) north of San Francisco, and grew to about 15 square miles (39 sq km) in just a few hours, the California Department of Forestry and Fire Protection said.

The blaze has since exploded to more than 101 sq miles (262 sq km) amid triple-digit temperatures and land parched from several years of drought. A thick layer of smoke kept air tankers and helicopters from flying for a time.

The fire forced the evacuation of two towns as well as residents along a 35-mile (56km) stretch of State Route 29.

The firefighters were airlifted to a hospital burns unit, where they were listed as being in a stable condition, department spokesman Daniel Berlant said.

The fire so far has destroyed 86 homes, 51 outbuildings and was threatening about 6,400 more.

"I lost my business - it's all burned up - my shop, my house, 28 years of living," said Joe Thomas, who lives near Mountain Ranch. "I got to start all over. It's depressing."

A choking fog of smoke and ash turned California's grassy, tree-studded Gold Country an eerie white. Away from the burned-out cars and smouldering remains of homes, Annette Stout and other residents who fled rested at evacuation centres.

"I grabbed my cats, their carriers, important papers, my husband's death certificate and his ashes," said Mrs Stout.

California Governor Jerry Brown declared a state of emergency and more than 3,850 firefighters were assigned to the blaze. Its cause is under investigation.