Firefighters were trying to get a handle on the blaze near the small community of Valley, about 40 miles north of Spokane. County Commissioner Merrill Ott estimated that 40 to 50 homes were in the fire's path.
Several aircraft were helping fight the fire and more were on order, he said. The only reported injury was a firefighter with a laceration.
The State Patrol closed a stretch of U.S. Highway 395 between Jump Off Joe Lake and Chewelah. Other roads were closed as well.
"Resources from all over the state will converge here," said Steve Harris, a spokesman for the state Department of Natural Resources. "It's going to take several days to get this thing looking really good."
In Idaho, a force of 767 firefighters faced tough terrain as wind-blown wildfires engulfed thousands of acres Tuesday.
"It's a running battle mainly because it's very rugged, steep and hard to get people in," said David Eaker of the Great Basin National Incident Management Team.
One fire burned an abandoned cabin and threatened 10 evacuated homes near Bonanza that have been wrapped in fireproofing material and rigged with roof sprinklers.
Elsewhere on the Salmon-Challis National Forest, there were 29 new lightning-sparked fires reported Tuesday. Crews were being sent in to try to prevent those fires from growing.
Firefighters in southern California were battling two brush fires that burned at least 1,000 acres in Riverside County and forced the evacuation of several vacation homes.
The larger blaze started about 2 p.m. Tuesday and quickly charred about 800 acres, or 1.2 square miles, of hilly terrain near San Jacinto, a community 80 miles east of Los Angeles.
Its rapid spread prompted fire officials to evacuate about 10 people staying in vacation cottages on a country club owned by the Soboba Band of Luiseno Indians. They also called for voluntary evacuations of some 40 homes in the area.
In north-central Washington, firefighters continued to battle four other large fires while keeping watch for storms that could bring winds and lightning. The fires had burned about 130 square miles.
Scattered thunderstorms forecast for eastern Washington were not expected to produce much rain, but lightning strikes could spark new blazes, the National Weather Service said.