The blaze was started by lightning Aug. 7. It expanded significantly Friday, burning through tinder-dry brush and timber in the mountains west of the towns. Efforts Saturday were focused on protecting homes and structures in the gulches and foothills just west of Hailey and Ketchum.
Crews also planned to target erratic, wind-driven fire growth in one area north of Ketchum and two locations west of Hailey.
For now, that battle is taking place on the ground because heavy smoke has grounded air tankers capable of dumping retardant and water on the perimeters.
Fire managers and more than 700 firefighters are bracing for another dose of hot temperatures and afternoon wind gusts up to 30 mph that could move flames closer to homes. Hundreds of more firefighters will be arriving Saturday as other fires in Idaho and other western states die down.
"This fire is consuming everything," fire spokeswoman Madonna Lengerich said. "The fire is so hot it's just cremating even the biggest trees."
The tight valley that bookends these communities and serve as vacation getaways for celebrities like Arnold Schwarzenegger and Tom Hanks is covered in thick smoke.
Downtown retail and dining districts, normally buzzing this time of year with tourists and summer residents, are quiet, and some merchants haven't even bothered to open for Saturday breakfast and lunch.
Ketchum, with a population of 2,700, and Sun Valley, with 1,400 people, also are under "pre-evacuation orders." Authorities have told them to be ready to leave if necessary.
On Friday, Sun Valley Ski Resort officials turned on water cannons, used for wintertime snowmaking, to protect lodges on the mountain and ski lift equipment.
In addition to the government firefighters, some private insurers have sent in their own crews to provide structural protection for homes with values that can stretch into the millions of dollars, Blaine County Sheriff Spokeswoman Bronwyn Nickel said.
"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."