A lightning-sparked blaze raged through a national forest recreation area in the mountains northwest of Las Vegas as more than 1,000 firefighters battled to regain lost ground on Wednesday.
The landscape burned by the fire has grown by 5,000 acres since Tuesday alone, while the containment around the fire zone slipped from 15 percent to 10 percent, said Suzanne Shelp, a spokeswoman for the incident management team.
The blaze has pumped smoke into the air over the greater Las Vegas metropolitan area, and charred more than 25,000 acres of pinyon-juniper woodlands since it was sparked by lightning on July 1, officials said.
Six buildings, including at least one commercial structure, were destroyed on Tuesday, officials said. It was not immediately known whether the others were residences or businesses.
No further structures were known to be in immediate danger on Wednesday.
The blaze was ranked as the top U.S. firefighting priority by federal authorities.
The fire spread on Tuesday mainly into areas that were less steep and difficult to reach than before, and the size of the firefighting force was increased by 200 personnel, Shelp said.
"So it was a good day and also a frustrating day," she added.
The blaze was one of over a dozen burning across the United States in what experts say could be one of the worst U.S. fire seasons on record.
In recent weeks a Colorado wildfire ranked as the state's most destructive on record ravaged more than 500 homes and killed two people, while 19 members of an elite "hotshots" crew battling a separate fire in Arizona died on the job on June 30.
In Nevada, at least 10 hotshot crews have been assigned to the Carpenter 1 blaze, out of about 100 such specially trained outfits available nationwide.
The homes of some 500 people who live within or on the fringes of the Spring Mountains National Recreation Area, a unit of the Toiyabe National Forest, were evacuated days ago, and no injuries have been reported.
The Spring Mountains recreation area, about 35 miles northwest of Las Vegas, is a popular outdoor getaway for residents of Nevada's biggest metropolitan area, used for hiking and camping year-round and skiing and sledding in winter.
The Spring Mountains, a forested, spring-fed oasis in the Mojave Desert of southern Nevada, is also known for its scenic beauty, including the state's third-highest mountain, Charleston Peak, and encompasses a vital watershed in the region.
The blaze erupted near the town of Pahrump, west of the recreation area, but has since burned into the mountains, fueled by tinder-dry vegetation and stoked by high winds, officials said.