A major wildfire is continuing to spread towards Denver, the capital of the US state of Colorado.

There is such an amount of heat that you can't put firefighters in front of (the fire)

Fire information officer

Hundreds of residents have left their homes southwest of Denver, and thousands more have been urged to leave.

The fire, stretching 24 kilometres (15 miles) across the Rocky Mountain foothills, has destroyed 21 homes and is threatening another 2,500, firefighters say.

By Tuesday evening, the flames were about 11 kilometres (seven miles) from Roxborough, on the far southwest edge of the metropolitan area of Denver and 56 kilometres (35 miles) from central Denver.

Shifting wind

Officials say the fire is nowhere close to being contained, and it is too dangerous to put firefighters on its northern fringes - between the flames and homes in Douglas County.

"There is such a tremendous amount of heat that you can't put firefighters on the ground in front of it," fire information officer Tony Diffenbaugh said.

Douglas County urged more than 13,000 residents near Sedalia - 20 miles (32 kilometres) south of Denver - to leave.

Earlier on Tuesday, shifting wind had helped slow the fire's march toward Denver.

The blaze is one of at least eight burning across Colorado - including a 4,240-hectare (10,600-acre) fire that destroyed 28 homes near Glenwood Springs, about 240 kilometres (150 miles) west of Denver.

Tinder

The Colorado Springs Gazette says the heat wave is expected to cool in the next few days but dry lightning is forecast for Wednesday - a further threat to the tinder-dry forests

Thousands of homes are threatened

"The trees in the forest are drier than what you'll find in a lumber store," said Terry McCann of the US Forest Service.

Firefighters have been using aircraft to try to douse the blazes, which have blanketed Denver and other areas with smoke and ash.

The National Guard and civilian volunteers have been helping fire crews to try to contain the flames.