The blaze is one of several burning in the US West as extreme heat makes it difficult for firefighters to tamp down the flames.
In Utah, new evacuations were ordered east of the fire's epicentre in the tiny mountain town of Brian Head, with residents nearby and people visiting a popular fishing area also asked to leave to avoid danger, said Denise Dastrup of the Garfield County Sheriff's Office.
The mountain bike and hiking trails, zip line and water tubing hill that lure summer visitors were closed while firefighters try to contain the blaze.
The resort, which is a ski area in the winter, says on its website that it still plans to carry out summer activities that include music concerts and Fourth of July events but that it doesn't know when it will reopen.
The unidentified person accused of starting the fire could face charges. Firefighting costs could rise to more than $1 million, said Jason Curry of the Utah Division of Forestry, Fire and State Lands.
Anita DeLelles, a cabin owner in Brian Head, said she's saddened, frustrated and angry that the action of one "careless person" is causing so much damage. She wasn't there when the fire started, but people renting her cabin had to be evacuated.
She said she's losing income from having to cancel reservations during busy summer months. But that's secondary to her concern for what the town will look like when the fire is finally out.
The fire has spread to 17 square miles (44 square kilometres) and is heading toward a lake used by recreationists and where some homes stand, said Erin Darboven of the Bureau of Land Management.
A stretch of closed highway has been extended to nearly 48 miles (77 kilometres).
Garity Hathaway was at the top of a mountain hiking trail Saturday with her husband and their two children when they saw the flames. They ran down a ski slope to get their things from their hotel and leave town, cutting their weekend getaway short by a day.
Another fire, sparked by lightning nearly three weeks ago, has charred more than 11 square miles (28 square kilometres), and crews have started rehabilitation work. With the hot, dry conditions persisting, forest officials imposed fire restrictions in hopes of limiting human-caused fires.
More than 1,200 firefighters got help Thursday from a fleet of water- and fire retardant-dropping aircraft. No buildings were threatened.
Firefighters in the San Francisco Bay Area brought a grass fire near a highway in Vallejo under control after it damaged several properties and led to major traffic backups.
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