All of Utah and much of Wyoming, Colorado and Montana were under a red flag warning, meaning conditions were hot, dry and ripe for fires.
The forecast for Denver called for a fifth straight day of 100-plus degree temperatures on Tuesday. Monday's high of 105 was a record for that date.
Other areas in the state have also been topping 100 degrees, including northern Colorado where the state's second largest wildfire in history is burning.
The scorching heat doesn't appear to be letting up soon. Segin said such prolonged heat is "extremely taxing" physically on firefighters, who are working long days and carrying heavy gear.
The 7-square-mile Waldo Canyon fire west of Colorado Springs, Colo., sent heavy smoke billowing over an upscale neighborhood as firefighters battled to keep the fire from burning houses and advancing toward the Air Force Academy.
The fire was less than 5 miles from the southwest corner of the academy's 28-square-mile campus, fire information officer Greg Heule said. Winds appeared to be pushing the fire to the west of the school.
Television video showed smoke and flames close to houses in a forested neighborhood northwest of Colorado Springs. There were no reports of homes burning.
The fire was 5 percent contained.
Two specially equipped Air Force C-130 cargo planes were helping fight the fire, and a third was expected later in the day.
With the nation's privately owned fleet of heavy air tankers already in use or unavailable, U.S. Forest Chief Tom Tidwell said his agency had to call on the military to help.
Tidwell told The Associated Press in a phone interview Monday that about half of the nation's personnel who are usually assigned to large fires are working in Colorado right now.
"It's just because it's so dry," Tidwell said. "Not unlike New Mexico they saw very low snowpack, especially in that lower country. Hot, dry winds with dry fuels, you get the ignition, and this is what we see."
At the 136-square-mile High Park fire in northern Colorado, authorities increased the number of homes destroyed to 257, saying they found nine homes that hadn't been counted earlier. The total was already the highest of any wildfire in state history.
That fire was 55 percent contained.
• A 60-square-mile fire in Utah burned more than two dozen homes, firefighters said, and they were beginning an inventory to see if other structures were lost. The fire was 15 percent contained.
• A fire that charred nearly 70 square miles west of Ruidoso, N.M., was 90 percent contained, with many residents allowed to return home.
• A wildfire north of Helena, Mont., destroyed four homes, and officials said another 150 could be under threat if windy weather fans the flames.