Arizona Blaze Firefighters

Fire crews working overnight protected most of a mountain-resort town threatened by an eastern-Arizona wildfire, while federal officials revised downward their estimate of the blaze's size.

Fire-information officer Suzanne Flory said overnight operations went well, particularly in the community of Greer, where the blaze had made a significant run Wednesday.

"Firefighters had a good stand and were able to protect the main part of town, but structures were lost. We don't know how many at this point," Ms. Flory said.

Federal officials released an updated size of the blaze based on better mapping. The fire has scorched 525 square miles, mostly in ponderosa pine forest, down from 607 square miles reported Wednesday. It remained the second-largest fire in Arizona's history.

Several mountain communities have emptied in advance of the fire, and a utility that supplies power to customers in southern New Mexico and West Texas warned of possible power interruptions. Losing the lines would cut off about 40% of the utility's supply, possibly triggering the rolling blackouts among its 372,000 customers.

In nearby New Mexico, many residents in the community of Luna said they chose to stay even after being told to prepare to flee. Many mowed or watered lawns and removed debris, while crews bulldozed lines and set backfires to build a border of fire protection.

At more than 600 square miles, the fire has burned a larger area than the cities of New York City and Chicago combined. And it will probably continue to grow.

A map of the unruly fire shows a bright red, multitentacled mass extending across the Apache and Sitgreaves National Forests. But Mr. Whittington pointed out the tiny red dots beyond the front lines of the fire representing spot fires that, if not controlled, would extend the main fire’s march to the northeast across the New Mexico state line.

One spot fire east of Alpine, Ariz., was already a quarter-mile across, Mr. Whittington said. The fire came so close to the mountain retreat of Greer on Wednesday afternoon that firefighters had to retreat for a while, officials said.

“We’re going to do our best to catch everything,” he said. “We want to dampen down any runs the fire wants to make.”

Despite the work of more than 2,500 firefighters, the Wallow Fire was zero percent contained on Thursday morning, meaning that it continued to have the potential to extend in any direction. The southern part of Eagar, a town at the edge of the White Mountains, was evacuated Tuesday afternoon; residents in the rest of that town and in neighboring Springerville were told to leave Wednesday night. Meanwhile, two additional high-priority firefighting teams were preparing to join the battle.

“I’m not going to say confident,” Mr. Whittington said of the current mood of those struggling to contain the blaze. “There’s no guarantee with fires.