Two people have been confirmed dead as fire crews fight to contain a destructive wildfire in the US state of Colorado.

The blaze has spread across more than 6,215 hectares of forested terrain northeast of Colorado Springs since it erupted.

About 38,000 people have been forced to leave their homes.

El Paso County Sheriff Terry Maketa said the dead were found in the garage of one of at least 360 homes destroyed by the so-called Black Forest Fire near the state's second-largest city.

Sheriff Maketa indicated the fire could be the work of an arsonist or the result of negligence, and said a criminal investigation was under way.

He said there were no signs of lightning strikes in the area when the fire began.

News of the first two casualties from the blaze came as firefighters made their first measurable progress against the fire, managing to carve containment lines around 5% of the fire's perimeter.

Sheriff Maketa said the victims had been on the phone as the flames closed in on their home.

"The person they were speaking with said he could hear popping and cracking in the background and they (the two people) advised they were leaving right now," he said.

"We were truly hoping that we could get from day to day without coming across news like this."

With the fire still burning largely unchecked and driven by erratic 48km/h winds that showed no sign of diminishing, officials yesterday ordered mandatory evacuations of about 1,000 homes in the northern tip of Colorado Springs that were considered to be in imminent danger.

A voluntary evacuation alert was issued for another 2,000 homes in Colorado Springs, advising those residents to be ready to leave at a moment's notice.

The Black Forest fire, named for the community near where it began, was the largest of several blazes burning across Colorado, underscoring concerns that persistent drought could intensify this year's fire season in the western US.

More than 400 firefighters were battling the fire, assisted by helicopters and planes equipped to drop water and fire-retardant chemicals on the flames, and 140 personnel from the Colorado National Guard.