Fire officials said high, gusty winds, low humidity and tinder-dry vegetation created unstable conditions surrounding the 85-square-mile Beaver Creek Fire.
One home was destroyed Thursday night, said Bronwyn Nickel, a spokeswoman for Blaine County, where the fire is burning.
More than 600 state and federal firefighters were dispatched to the blaze in the affluent resort region that's a second home to celebrities such as Arnold Schwarzenegger, Tom Hanks and Bruce Willis.
In addition, private insurers have sent in their own crews to provide structural protection for homes with values that can reach tens of millions of dollars, Nickel said. Similar measures were taken during a 2007 blaze near Ketchum.
"There are private engines that insurance companies have sent in," she said. "They're on site, they're working with our local firefighters and law enforcement."
A huge DC-10 tanker, capable of carrying 12,000 gallons of retardant, was among aircraft making drops on the blaze.
Flying in separate aircraft, Gov. C.L. "Butch" Otter and U.S. Forest Service Chief Tom Tidwell took an aerial tour of the fire.
The towns of Ketchum, with a population of 2,700, and Sun Valley, with 1,400 people, were under pre-evacuation orders, with residents advised to prepare their belongings in case they are required to leave on a moment's notice.
Fire managers "are just adding an extra layer of caution to the plan that they started last night," said Rudy Evenson, a spokesman for the federal team overseeing the fire. "We have a forecast for 30 mph winds at the ridge tops. "
Southbound traffic on U.S. Highway 75 was backed up, as many residents and vacationers opted to flee the smoke.
"They just don't know which way the fire is going to go, so they've got to get everybody ready," Hailey resident Jane McCann said. "The smoke is unbearable. Today in Hailey, you couldn't see the mountains from Main Street."
McCann said traffic on US 75 was bumper-to-bumper.
The Beaver Creek Fire was burning into an area torched by the 2007 Castle Rock Fire that burned a portion of the Sun Valley ski area on Bald Mountain. The scarred terrain from that fire remains a significant buffer to this latest blaze, though flames that reach heavily timbered patches on south-facing slopes were expected to burn intensely, Evenson said.
In addition to assessing the Beaver Creek Fire, Otter and Tidwell were expected to survey the Elk Complex and Pony Complex fires to the southwest, where firefighters were getting the upper hand a week after flames torched dozens of cabins and other structures.
The Elmore County Sheriff's Office was escorting homeowners from Fall Creek, as well as Lester Creek and Ice Springs, back into their neighborhoods. But they were only allowed a limited amount of time to survey damage to their properties, due to safety concerns.
Charred slopes were unstable, and crews have already spent days cutting through downed trees.
Both fires were closer to containment, though the remote mountain hamlets of Pine and Featherville remained under evacuation orders.
Firefighters battling the Elk Complex were conducting burnout operations after the blaze -- estimated at 200 square miles along the Pine/Featherville Road -- was about 40 percent contained.
The Pony Complex -- the largest in Idaho so far this year at 229 square miles ---- was 80 percent contained. Crews worked Thursday to extinguish remaining hot spots near Anderson Ranch Dam.
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