"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."
In Utah, the Patch Springs Fire burning in Tooele County jumped Highway 199 as it burned through the community of Willow Springs, about 60 miles southwest of Salt Lake City. That fire was sparked by lightning Aug. 10.
Meanwhile, firefighters reported progress on a fire burning near Park City, Utah.
The Rockport broke out Tuesday and has destroyed eight homes. Officials say it has burned about 1,900 acres and was 58 percent contained early Saturday.