A fast-burning fire that quadrupled in size in the matter of hours threatened a small southwestern Utah town, while firefighters made progress on a separate wildfire that had blackened almost 70,000 acres.

Causing the most concern was a fire burning about 20 miles north of St. George. Officials said the fire grew from 2,000 acres to 8,000 acres in less than 12 hours, and by Monday was within four miles of New Harmony.

The blaze was one of 19 active fires burning 500,000 acres in Alaska, Arizona, California, Idaho, Nevada, New Mexico and Utah, according to the National Interagency Fire Center.

By Monday, firefighters had the blaze near New Harmony about 30 percent contained, but officials warned those gains could be quickly erased if expected high winds arise.

Fire crews had not yet mapped the fire but said it continued to grow overnight beyond the 8,000-acre estimate given late Sunday, said Dave Boyd, a fire information officer for the Bureau of Land Management (search).

Craig and Leah Twitchell stood on the back porch of their New Harmony home early Monday, watching the fire burn on the mountain just south of them.

Against the black night sky, Twitchell said he could see two bright lines of fire; one moving north and another moving east. "It's scarily pretty," said Twitchell, a retired state trooper who has lived in town about five years.

The fire with 10-foot tall flames was sparked Saturday by lightning strikes, and at one point jumped the interstate. State officials closed the highway, the major route between Salt Lake City and Las Vegas, for about seven hours overnight Saturday, and again briefly late Sunday to allow helicopters to drop retardant.

About 20 miles southwest, the state's largest fire scorched 68,264 acres, though firefighters were able to keep the blaze from growing thanks to winds of less than 10 mph. It was expected to be fully contained by Tuesday.

In Nevada, firefighters believed they had a handle on a 33,500-acre wildfire that had blanketed Las Vegas with smoke and prompted the evacuation a Boy Scout camp.

Fire information officer Joe Colwell said the blaze southwest of Las Vegas was 60 percent contained on Monday and could be fully contained by Tuesday night.

"We're basically in a mop-up mode now," he said. "But we still have pretty explosive conditions."

Forecasts called for hot, dry conditions Monday, with increasing winds and a chance of thunderstorms on Tuesday. Winds were expected to diminish Wednesday.

In California, firefighters were close to containing a massive fire in the Mojave National Preserve (search) that blackened some 67,000 acres and destroyed five homes and two cabins built in the late 1800s.

Milder-than-expected winds from a cold front and steep terrain helped slow down the fire, which was 65 percent contained by late Sunday. Full containment was expected Monday.

"It was still hot but the winds were mild. And the humidity was still low but it wasn't down in the single digits," said Capt. Greg Cleveland, a spokesman with the Southern California Incident Management Team (search).

The fire formed after lightning sparked five separate blazes Wednesday afternoon near the Nevada state line. Those smaller blazes eventually merged and were burning at the edges of critical territory for the threatened desert tortoise.

The fire scorched sections of the preserve containing historic mines and sites of ancient Indian pictographs, but the extent of the damage was unknown. Six trailers or other structures also were destroyed.

In Arizona, a wildfire burning northeast of Phoenix had consumed 92,145 acres by Sunday night. The fire was growing to the north and northeast, away from populated areas that had initially been threatened by the lightning-sparked fire.

In Washington, a wildfire burning grass and wheat fields on farmland in Walla Walla County had blackened 22,000-plus acres by Sunday night, with smoke from the fire reported as far as Spokane, about 100 miles north.