"We haven't lost any homes, but it's real close," said State Forester Bill Crapser.
He said officials were concerned that strong winds could drive flames toward houses on the north face and top of Casper Mountain, about five miles south of town. The lightning-sparked wildfire had already burned 8,000 acres, or about 13 square miles, since Monday. Gov. Dave Freudenthal declared a state of emergency on Tuesday.
About 200 firefighters were assigned to the fire Wednesday morning, and that number was expected to increase during the day, Crapser said.
He said about half the firefighters would be assigned to protect houses while the rest cleared fire breaks to stop the flames' advance. An estimated 300 homes in the heavily forested area were ordered evacuated Monday and Tuesday.
Officials at the National Interagency Fire Center in Boise, Idaho, said all available equipment and crews are fully engaged on more than 40 large wildfires across the West.
Idaho led the nation with 12 large wildfires Tuesday, including several on the fringes of rural mountain communities.
The state Department of Environmental Quality issued what it said was its first "red" air quality alert for the populous Boise Valley after drifting smoke from wildfires sent ozone pollution to unhealthy levels.
Elsewhere, 150 firefighters were battling a lightning-sparked wildfire that had grown to 20,000 acres in Elko County, Nev. Army troops were being sent to two fires that have blackened more than 140 square miles in northern Washington state.
Nationwide, more than 6.3 million acres have burned this year, well above the 10-year average of less than 4 million acres burned by this time of year, according to the NIFC.
"We're focusing on protecting community infrastructure, historical resources and precious watersheds," said Rose Davis, a Forest Service spokeswoman at the NIFC. "We need to look at where we can be the most effective with what we have, knowing these fires could keep burning for another month or so."