The 350,000-acre Park fire in Northern California was still burning on Sunday, even as the authorities said that falling temperatures and higher humidity were reducing fire activity.

More than 25,000 firefighters and other personnel have been working to tame active blazes that have scorched more than two million acres across the United States. Smoke and haze from wildfires in Canada and the American West have traveled as far east as New England and South Carolina.

The Park fire near Chico, Calif., which started on Wednesday, has traveled across four counties and was the largest fire burning in the United States on Saturday. It was caused by arson, according to the California Department of Forestry and Fire Protection.

The fire was only 10 percent contained as of Saturday night, and nearly 4,000 firefighters and other personnel were trying to contain it, CalFire said. At least 20 structures had been destroyed.

About 4,400 people were under an evacuation order within the unincorporated areas of Butte County, Kory Honea, the county sheriff, said at a Saturday news conference. Lassen Volcanic National Park was closed and evacuated on Saturday as the fire approached the western edge of the park.