COLORADO SPRINGS - Firefighters struggled on Wednesday to beat back a fiercely aggressive wildfire raging at the edge of Colorado Springs, Colorado, that has forced at least 35000 people from their homes and was nipping at the edges of the US Air Force Academy.

The so-called Waldo Canyon fire, fanned by gusting winds, has gutted an unknown number of homes on the wooded fringes of Colorado's second most populous city and prompted more evacuations as flames roared out of control for a fifth day.

President Barack Obama planned to pay a visit to the area on Friday to view the damage, the White House said.

The blaze flared on Tuesday night with sudden ferocity and quickly overran fire containment lines, invading the northwestern corner of the city. But officials have declined to characterise the extent of property damage there.

Colorado Springs fire chief Rich Brown said he had "a number in mind" for the approximate tally of homes lost in Tuesday night's firestorm but did not want to release that information until residents of those dwellings were notified.

Governor John Hickenlooper said after flying over the fire zone on Tuesday night that he saw many homes destroyed in a "surreal", luminous landscape below, but did not give a number.

Mr Brown called the firestorm threatening his city - in a metropolitan area of more than 650000 people - "a monster" and said that flames were "not even remotely close to being contained".

The fire, which has scorched more than 6070ha in the shadow of the famed Pikes Peak mountaintop, was not as intense early on Wednesday as the day before. But thunderstorms hampered firefighting efforts by bringing erratic winds, whipping flames from one direction to another.

The number of evacuees stood at 32000 on Tuesday night, but after additional residents in and around the nearby town of Woodland Park, population 7600, were ordered from their homes on Wednesday, El Paso County Sheriff Terry Maketa said the number of displaced people had grown by about 3000.

Colorado wildfires have killed four people so far this year in what the governor called "the worst fire season in the history of Colorado", but no serious injuries from the Waldo fire were reported on Wednesday.

The US Air Force said it was dispatching up to 25 military helicopters, joining four giant C-130 air tankers already devoted to the effort. More than half of federal firefighting resources are deployed in Colorado, the White House said.

The Waldo Canyon fire was one of about a dozen burning across Colorado. The biggest of those, the High Park fire near Fort Collins in the northern part of the state, has consumed 35300ha but was 65% contained. It now ranks as the most destructive wildfire on record in Colorado after devouring about 250 homes.

Elsewhere across the Rocky Mountain West, a wildfire in Montana that has scorched 7690ha in the eastern part of the state forced 600 residents to leave their homes, said Kent Atwood, a state hazard mitigation officer. An unknown number of houses were destroyed.

In Utah, authorities found a body in the ashes of the fast-moving Wood Hollow fire about 160km south of Salt Lake City on Tuesday, marking the first fatality in a blaze that has scorched more than 18692ha of rolling hills.