The largest fire to strike Tahoe National Forest this year the American fire has now burned an estimated 11,950 acres.
Steep terrain and a forecast calling for higher winds are causing concern that the fire will spread, said Mark Brown, spokesman for the U.S. Forest Service. The fire was about 45 percent contained Saturday, he said.
The fire, whose origin is under investigation, broke out 17 miles northeast of the town of Foresthill on Aug. 10 and is spreading to the southeast after making gains to the northeast on Friday, Brown said.
"Right now, the fire is burning in some steep canyons at the southeast ridge," he said.
No populated areas are threatened.
Crews hiked two miles into the forest after leaving Deadwood Ridge Road, where the fire in burning to the south.
"They're having some significant containment problems there right now," Brown said.
An emergency closure order is in place for portions of National Forest System lands within and adjacent to the American fire.
So far, the fire has demanded the use of 1,396 firefighters and eight helicopters.
"I would not be surprised if that number gets bigger," Brown said.
The California Department of Forestry and Fire Protection has supplied roughly 200 of those firefighters, said spokesman Daniel Berlant.
"In the Sacramento region and foothills, this is the largest fire we've seen this year," Berlant said.
He said fire activity in the region has increased in the past two days. That includes a 100- acre fire that forced evacuations in El Dorado County on Friday and a 1,000-acre fire in Butte County that also has demanded evacuations.
Several small fires are also burning in Placer County, Berlant said.
"Our region has definitely seen an increase in fires, and that is only going to continue as we start to see a weather system move into our region that's bringing in some pretty strong winds," Berlant said. "That will increase the fire danger as well as the risk for dry lightning."
Berlant said that fire danger is high to extreme in most foothill and mountain areas and that the threat will go higher after this weekend as the weather shifts.
Call The Bee's Edward Ortiz, (916) 321-1071. Follow him on Twitter @edwardortiz.