About 4,000 people have fled their homes and officials have drawn up evacuation plans for up to 40,000 residents of the state's capital, Denver, as the fire approaches the city's south-western suburbs.

The fire - stretching 24 kilometres across the Rocky Mountain foothills - was started by a camp fire on Sunday

In some areas even firefighters were told it was too dangerous to work

Officials said the fire was devouring about one-and-a-half kilometres of woodland an hour

They said the fire destroyed 21 homes and was threatening another 2,500

Emergency plans were put in place to evacuate residents of Denver if the blazes continued its destructive course

Colorado's governor Bill Owens said the disaster area resembled a nuclear winter

Firefighters tried to limit the spread of another major fire started by an underground coal fire

It is estimated that recent fires in Colorado has destroyed more than 600,000 acres (24820 hectares) of land