An out-of-control wildfire zeroed in on hundreds of rural dwellings in the rugged Sierra Nevada on Tuesday after leveling nearly 100 homes and forcing the evacuation of at least 8,000 people.

The warm, dry winds that drove the fire across 40 square miles of terrain covered with timber and brush continued this morning as sleep-starved firefighters struggled to hold their own against the 70-foot-high flames."We probably have as extreme a condition as anywhere in California or anywhere in the U.S. today," said operations director Ed Waggoner of the state Department of Forestry.

At least 149 structures, including 92 homes, were destroyed in an area about 60 miles northeast of Sacramento, authorities said. Some of the homes were valued at $1 million.

Fire officials stressed that many more houses are still in extreme danger, especially Rough and Ready, a community of about 500 residents - all evacuated - in the heavily wooded bottom of a steep canyon on the edge of the fire.

Fire planning chief Steve Iverson said saving Rough and Ready was one of the top priorities today, but he cautioned that it was a particularly dangerous area for firefighters because of the limited access on a narrow road.

"If we have shifty winds, we're not going to sit around in there," Iverson said.

A preliminary estimate by the U.S. Forest Service put the loss in structures alone at more than $9 million, and the figure was expected to climb much higher.

"I was coming back from the Bahamas, sitting in San Francisco International Airport, and I see my father's house burning on television," said insurance agent Don Sanzone.

Jim Mowner, a state forestry spokesman, said the continued stiff winds and low humidity were hampering firefighting.

"I would say the fire will burn actively overnight," he said.

Before dawn, Mowner's prediction was borne out, as Department of Forestry officials reported that the fire had grown overnight from 25 to 40 square miles. The only good news, Mowner said, was that the fire was spreading into an area east of Beale Air Force Base, where there were fewer homes.

"That could help us," he said. "We've been spending an awful lot of time and manpower protecting structures, so the main fire has been burning relatively freely."

More than 1,000 firefighters had been added to the lines Monday and Tuesday, bringing the total to nearly 2,800 battling the blaze, said forestry department spokeswoman Karen Finlayson.

The blaze began early Sunday when a transient set fire to toilet paper. The unidentified man was cited for burning without a permit and released, authorities said.

Meanwhile, firefighters in Yellowstone National Park took advantage of a cold spell to make some headway against blazes that have devastated much of the famed park before hotter, drier weather returns.

"The fire people realize that things could heat up again this week," said fire information officer Sonya Capek. "The fires are still a threat; we are not out of it yet. We know there is some more fire time left in the season, but hopefully it won't be as intense."

Temperatures were only in the 40s this morning while the force of some 8,000 firefighters worked to strengthen existing firebreaks and prepare to protect the areas of Mammoth Hot Springs, and Gardiner and Jardine, Mont., if necessary. Jardine and park headquarters at Mammoth were evacuated before rain and snow fell over the weekend.

President Reagan said Tuesday that the Interior Department policy of letting naturally caused fires burn in national forests had been scrapped in the wake of the Yellowstone fires.

Asked what he thought of the policy, Reagan told reporters, "I have questioned it. I did not even know it existed, frankly." He said that Interior Secretary Donald Hodel has "made it plain that we were withdrawing from this policy."

About 4,500 residents of the affluent Lake Wildwood area were forced from their homes Sunday and Monday as erratic winds shifted the blaze. At least 3,500 people have been evacuated from the nearby communities of Smartville, Rough and Ready, and Penn Valley, since Sunday.

Two firefighters suffered second- and third-degree burns. Their conditions were not immediately available.

The fire jumped Highway 20 and headed southwest on Monday, threatening hundreds of scattered, rural dwellings and forcing the latest round of evacuations.

"There are hundreds here and they are pouring in," said Red Cross worker Elizabeth Quirk at the Nevada Union High School evacuation center. "The school buses are bringing in senior citizens, and there are campers and motor homes in the parking lot."