Crews backed by important firefighting resources are gaining ground against a massive Northern California wildfire, but it may be several days before thousands of evacuees can return home, officials said Thursday.

Four more homes were destroyed overnight, bringing the total number of properties lost to 43. More than 13,000 people have been ordered or urged to leave their homes, vacation cabins and campsites since the blaze ignited July 29 about 100 miles north of San Francisco.

Dominic Polito, a fire spokesman, said he does not know when residents will be allowed back home.

"I would imagine they're going to make sure the line is completely contained before they do that," Polito said Thursday. "Plan on the next couple of days, and hopefully it's sooner than that."

Firefighters and equipment from outside the drought-stricken state have poured in to battle the blaze, which has chewed through more than 107 square miles of parched terrain and was 40 percent contained, the California Department of Forestry and Fire Protection said.

Days after declaring a state of emergency, California Gov. Jerry Brown visited fire crews Thursday and thanked them for their efforts. He said the state is hotter and drier than it's ever been, making blazes more severe and extending the fire season.

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.

Across the U.S., 118 fires are burning on 2,757 square miles, according to the 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.

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.

Firefighters in Northern California have been working 24-hour shifts. They bunk in tight sleepers and depart in the mornings with enormous high-calorie sack lunches of sandwiches and cookies as others come back tired, footsore and hungry.

Some of the 3,400 firefighters on the blaze have been here since it started a week ago. The cause of the fire, burning in rugged terrain in Lake, Yolo and Colusa counties, 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. "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 Wednesday, freshly returned firefighters sat at long tables, telling jokes and digging into bowls of fruit, corned beef hash and fried eggs.

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.