A wildfire raging through the foothills and canyons of northern California's coastal mountains more than doubled in size as it roared into its sixth day on Monday, leaving two dozen homes in charred ruins and displacing thousands of residents.

The blaze, which has scorched about 54,000 acres east of Lower Lake, a town about 110 miles north of San Francisco, was the fiercest of 20 large fires being battled by 9,000 firefighters across the state, officials said.

A separate blaze that killed a U.S. forest ranger on Thursday near the Oregon border has also expanded, but remains a fraction of the size of the so-called Rocky Fire that erupted in Lake County on Wednesday and has proved the most destructive.

"This is a very fast-moving wildfire," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Prevention, or Cal Fire.

Some 20,000 acres of scrub oak and brush ravaged by the fire over a five-hour period on Saturday night represented "unprecedented growth in that short amount of time," he added. By Sunday evening, the blaze had blackened another 7,000 acres along the rugged eastern flanks of California's Northern Coast Ranges, officials said.

After destroying 24 homes and 26 outbuildings last week, the fire continued to threaten an estimated 6,300 structures and has forced the closure of parts of two state highways, Cal Fire said.

More than 12,000 people have received mandatory evacuation orders or advisories, while ground crews have managed to carve containment lines around just 5 percent of the fire's perimeter in the past two days, officials said.

Around 2,700 personnel were battling the Rocky Fire alone by Sunday night, about a third of the state's total force.

Cal Fire Chief Ken Pimlott said "well over 20 wildfires" were roaring across the drought-parched state following thousands of lightning strikes in recent days.

"We're certainly stretching our resources," he told CNN on Sunday, adding that National Guard troops had been mobilized, along with reinforcements from other states and the U.S. Forest Service.

A Forest Service firefighter from South Dakota, David Ruhl, 38, died on Thursday in the Frog Fire raging through Modoc National Forest near California's border with Oregon.

That blaze, which was about 4 percent contained on Sunday, has devoured 3,900 acres as erratic winds pushed the flames in all directions, the Forest Service reported.