The High Park fire had scorched 14,000 acres, destroyed 18 structures and prompted evacuation notices to about 1,600 people in northern Colorado.
"We're going to throw everything at it we can," Gov. John Hickenlooper said.
The wildfire, which started from an unknown cause early Saturday, was churning through the mountains about 15 miles northwest of Fort Collins, prompting authorities to urge people to leave.
"Sheriff showed up at the door step, pounding on my door, telling everybody to get out," one unnamed evacuee told CBS News. "So it was at that point I left. Been kind of out here ever since."
The Denver Post reported four helicopters and two air tankers were dropping water and flame retardant, with 250 firefighters and 15 engines working the uncontrolled fire on the ground.
Christensen said the fire was expected to be designated a Type 1 fire Monday, making it a top priority for national resources, the Post said.
Larimer County sheriff's spokesman John Schulz said the county had made a total 1,600 evacuation calls, including 800 Sunday in Poudre, Rist, Redstone and Mill canyons.
One firefighter was treated heat exhaustion, but no other injuries were reported and officials said there were no confirmed missing person cases.
The sheriff's department told KMGH-TV, Denver, two area residents were unaccounted for. However, Nick Christensen of the Larimer County Sheriff's Office said the number missing was uncertain.
"We can't say whether there will be one, 10 or a number in between there," Christensen. "We're hoping that it's none."
Sheriff Justin Smith said Saturday night it was "the fire we've always feared we'd have."
In New Mexico, the Little Bear fire was uncontained and had grown to more than 26,000 acres Sunday, CBS said. At least 20 structures were damaged and scores of residents had fled their homes, the TV network said.
Gov. Susana Martinez deployed the National Guard during the weekend to help with the fire burning in a remote and rocky region of the Lincoln National Forest, KOAT-TV, Albuquerque, said.
"I have asked the Guard to help local officials provide the security necessary to keep people around the area safe," Martinez said Saturday. "It is important that people remain ready to evacuate when asked and abide by the closures that are intended to keep people safe."
About 100 troops and two helicopters were assigned to the fire.