Backed by important firefighting resources, crews are gaining ground against a massive Northern California blaze that has destroyed numerous buildings and driven thousands of people from their homes, officials said.

The wildfire that has raged across more than 100 square miles of parched terrain was 30 percent contained Wednesday, the California Department of Forestry and Fire Protection said.

As firefighters and equipment from outside the state poured in to battle the blaze burning about 100 miles north of San Francisco, more than 13,000 people were required or urged to leave their homes, vacation cabins and campsites. At least 39 homes have been destroyed.

Aiding in the fight has been the preparation.

Fire officials got ready for a drought-fueled fire season and built up staffs early with several hundred more firefighters than previous years, Cal Fire spokesman Daniel Berlant said.

"We're definitely at a medium to high level of activity but we're not at extreme, where we are low on resources by any means," he said.

The National Interagency Fire Center in Idaho has listed the fire as the nation's highest priority for crews and equipment. It is the largest of 23 fires statewide and takes up nearly a third of the 10,000 firefighters dispatched in California, which has become tinder box amid years of drought.

Across the U.S., 118 fires are burning on 2,757 square miles, according to the Idaho fire center. About 17,200 people are fighting those fires, but resources are not tapped out yet, center spokeswoman Robyn Broyles said. If civilian crews run low, officials can call on national guard and military crews.

Firefighters come from near and far, working 24-hour shifts to snuff out an unpredictable blaze.

They bunk in tight sleepers and eat in a big mess hall. They depart in the mornings with enormous high-calorie sack lunches of sandwiches and cookies as others come back tired, footsore and hungry to their makeshift base at the Lake County fairgrounds.

August is the height of fire season, and while the number of fires nationally is below average, the 9,361 square miles burned to date is about 50 percent above average. Most of that — 7,731 square miles — has been in Alaska.

Some of the 3,400 firefighters on the blaze have been here since it started a week ago. The fire, burning in rugged terrain in Lake, Yolo and Colusa counties, is the biggest in the state and has destroyed at least 39 homes. Its cause is under investigation.

As much as crews love the work, fatigue inevitably sets in.

"This is our seventh day," said Seaton King, a battalion chief with the Pasadena Fire Department. He returned from a shift protecting structures and cutting low tree limbs.

"You kind of get used to it, but it's still taxing in terms of being up and available for those 24-hour work cycles."

At the fire camp here Wednesday, freshly returned firefighters in blue sat at long tables and dug into bowls of fruit, corned beef hash and fried eggs. They told jokes and drank coffee named after Justin Bieber, Johnny Cash and Metallica, to indicate strength.

Fire trucks parked outside hailed from Long Beach, Los Angeles and Riverside. Nearby were large white mobile sleepers that could fit 45 weary firefighters. Some preferred to pitch a tent.

"It's tiring work, no doubt about it. But most of these guys are in really great shape, and they thrive in this environment," said Hugo Patino, Modesto Fire Department battalion chief.

Mike Burt with the Glendale Fire Department in Los Angeles County said his crew received a call to help last Thursday morning and was in Lakeport that afternoon.

"I'd like to put my feet in an ice chest basically, but I won't do that," he said, laughing. "That would feel pretty good."

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Associated Press writers Kristin J. Bender in San Francisco, Nicholas K. Geranios in Spokane, Washington, and Jeff Barnard in Grants Pass, Oregon, contributed to this report.