An explosive wildfire ripped through 60,000 acres of forest in eastern Arizona Thursday, crossing into the town of Pinedale and coming within striking distance of two other communities, officials said.

It was unclear how much damage the fire had caused in Pinedale, a community about 10 miles west of Show Low that had been evacuated since Wednesday, said Kartha Icenhour, a fire prevention officer for the Apache-Sitgreaves National Forest.

The fire had crossed state Highway 260 and was heading north, Icenhour said.

Earlier, authorities had said the fire had begun burning structures on the outskirts of the town.

"This is a monster," said Jim Paxon, a U.S. Forest Service fire information officer.

About 250 firefighters dug fire lines on the Rodeo fire's east and west flanks but were unable to halt the northern edge of the fire as it scorched its way through Ponderosa pines and pinon juniper. It was also heading toward the towns of Linden and Clay Springs. Up to 4,000 people have been asked to evacuate the three communities.

Another 13,900 residents in the region were put on alert that they may have to leave on an hour's notice because of the Rodeo fire and a second blaze that began nearby on Thursday. Fire information officer Dorman McGann said the new, 150-acre fire was started by a woman lost in the woods, but other details were not immediately provided.

Residents of nearby Show Low, a town of 7,700 about 125 miles northeast of Phoenix, and neighboring Pinetop-Lakeside, where an additional 3,500 people live, were told to be ready to evacuate on an hour's notice. However, Paxon said that if the fire continued to move north it could bypass Show Low.

"It makes me a little nervous to know it's coming and there's nothing we can do about it," Show Low resident Ben Butler said early Thursday as he packed legal documents into his car. He said he and his wife had stayed up all night preparing to leave.

Authorities didn't know how the fire started but noted that without lightning in the area, it had to be caused by human activity.

The fire was one of 19 major blazes burning across the nation Thursday, according to the National Interagency Fire Center in Boise, Idaho. Overall, wildfires have scorched 1.84 million acres so far this year, more than double the 10-year average through this time of the year.

In Colorado, veteran Forest Service worker Terry Barton pleaded innocent to deliberately setting a wildfire southwest of Denver that has grown to 136,000 acres, destroyed at least 25 houses and forced 7,500 people from their homes since June 8.

The charges were filed after prosecutors expressed doubt in her story that the fire got out of hand as she burned a letter from her estranged husband.

The fire grew at a slower pace Wednesday, thanks to higher humidity and cooler temperatures, and similar weather was forecast for Thursday. Rain fell through the night in the Denver area and on parts of the fire, although it wasn't nearly enough to put it out.

"The main thing is that it will raise humidities and cool temperatures," fire information officer Dan Kincaid said. "We should be able to get in there, hopefully (Thursday) and work on line construction."

The blaze was just 40 percent contained.

The Arizona fire began Tuesday about a quarter-mile northeast of Cibeque, growing to about 1,200 acres by midday Wednesday before exploding. Gary Butler, sheriff in Navajo County, said authorities had been expecting a blaze of this magnitude because of the dry conditions in the state's forests.

"It wasn't if it was going to happen, but when it was going to happen," he said. "We just knew the big one was coming."

Gov. Jane Hull declared a state of emergency to free up funds to fight the blaze.

Meanwhile, local fire officials sprayed down homes to try to keep them safe.

There also were evacuations in New Mexico, where authorities went door-to-door in two canyons near Rociada, northeast of Santa Fe, to clear 21 families from their homes in advance of a 4,390-acre fire burning in the Pecos Wilderness.

In southwestern Colorado, near Durango, firefighters battling a fast-growing 54,000-acre blaze were focused on saving homes.

The fire, burning since June 9, grew by more than 9,500 acres Wednesday, jumping containment lines and threatening more than 400 homes. About 33 homes and 26 other structures have been destroyed and more than 1,760 homes have been evacuated.

A third Colorado fire, just 70 miles from Durango, forced 2,000 people out of their homes Wednesday near the community of South Fork. Half of the people were allowed to return home Thursday, but the fire had grown to more than 5,000 acres and destroyed at least eight homes, authorities said.

In California, cooler temperatures and calmer wind helped firefighters battling multiple blazes that have led to the deaths of an air tanker flight crew and the destruction of 13 homes.

Wind had died down at the scene of the wildfire near Yosemite National Park where three firefighters were killed Monday in an air tanker crash. The blaze, which began on Saturday, has blackened nearly 22,000 acres.

The Associated Press contributed to this report.