DENVER?Some 3,500 people were evacuated after a rushing wildfire swept through part of the western US state of Colorado, destroying a number of homes, officials said on Tuesday.

Local emergency authorities drafted in help from neighboring states, as the blaze spread to cover some 7,120 acres, while officials said more people might have to be evacuated.

"We've literally exhausted our resources," said local Boulder County Sheriff Joe Pelle, adding that federal firefighters were being dispatched to the area from Wyoming and New Mexico.

"Dozens of structures" had been destroyed so far by the blaze, which erupted Monday in Fourmile Canyon in Boulder County north of the state capital Denver, fueled by baking heat and dry conditions.

Pelle, who toured the affected area with Colorado Governor Bill Ritter, said nine volunteer firefighters who were battling the blaze had lost their homes.

Residents already evacuated were not being allowed back in to the area, he added.

Officials hoped to get air tankers over the area to help douse the flames, and were heartened that light winds were forecast down sharply from the 70 kilometer (45 mile) per hour winds that whipped up flames on Monday.

While the area has not been hard hit this summer by wildfires, officials were concerned that with conditions so dry the region had become a tinderbox.

The exact cause of the fire has not yet been determined, he said. Tuesday?s goal was to hold the fire line's perimeter and an investigative team was on standby to start gathering information on the cause.

The Colorado Department of Health issued a smoke health warning for Boulder County. Haze from the fire could be seen for miles away.