Tens of thousands of Colorado Springs residents forced from their homes by a raging wildfire took refuge with friends or family and crammed into hotels and shelters as Army troops helped firefighters protect the US Air Force Academy from encroaching flames.

The White House said President Barack Obama will tour fire-stricken areas of Colorado on Friday and thank firefighters battling some of the worst fires to hit the American West in decades.

Colorado Springs Police Chief Peter Carey said Obama's visit to Colorado, considered a key battleground state in the presidential election, would not tax the city's already-strained police force. Colorado Governor John Hickenlooper said he expected the president might sign a disaster declaration that would allow for more federal aid.

Fire from the Waldo Canyon wildfire burns as it moved into subdivisions and destroyed homes in Colorado Springs, on Tuesday. (Gaylon Wampler/Associated Press)

The blaze was burning out of control early Thursday in the mountains and within Colorado's second-largest city, after more than 30,000 evacuees frantically packed up belongings and fled.

The wildfire was one of many burning across the parched West, blazes that have destroyed structures and prompted evacuations in Montana and Utah and forced the closure of a portion of Zion National Park.

Thursday's forecast offered some hope, with the temperature expected to be about five degrees cooler than Wednesday — and humidity 15 to 20 per cent, about five percentage points higher.

Winds were forecast to be 15 to 15 km/h out of the west.

"It's not windy yet this morning — that's always a good sign," fire information officer Rob Dyerberg said Thursday.

Shifting winds Wednesday challenged firefighters trying to contain the approximate 75-square-kilometre Waldo Canyon blaze and extinguish hot spots inside Colorado Spring's western suburbs. The National Weather Service reported almost 100 kilometre-per-hour winds and lightning above the fire Wednesday afternoon, but winds were calmer by nightfall.

"It won't stay in the same place," said incident commander Rich Harvey.

Neighborhoods where explosions of bright orange flame were still dangerous Tuesday signaling yet another house had been claimed, keeping authorities away from being able to assess the damage.

'It won't stay in the same place.'—Incident Commander Rich Harvey

But an Associated Press aerial photo taken Wednesday of one neighborhood showed dozens of heavily damaged or destroyed homes.

Ed and Florine Gigandet took refuge in a hotel in Manitou Springs, which days earlier had been evacuated when the same fire passed through. They fled their home as ash fell on their driveway from an ominous orange smoke overhead.

Trying to learn about damage, the Gigandets drove to near their west Colorado Springs neighborhood to talk to police officers and see the area. They scoured media photos and spent hours on the phone with friends for any scrap of information. Authorities told the Gigandets it could be at least week before they're allowed home.

A helicopter battles the Waldo Canyon Fire by Air Force Academy on Wednesday, near Colorado Springs. The wildfire doubled in size overnight and has forced evacuations for more than 32,000 residents. (RJ Sangosti/Associated Press)

"We only packed clothes for four days," Florine Gigandet, 83, a retired photo printer, said. "I really thought that we'd be gone for only a day."

The Gigandets, avid golfers, left their clubs behind. "We should be out golfing," said Ed Gigandet, 81, a retired mining machinery sales analyst.

The fire burned about 10 acres along the southwest boundary of the Air Force Academy campus. No injuries or damage to structures — including the iconic Cadet Chapel — were reported.

Late Wednesday night, Air Force Academy officials announced they were relocating about 550 cadets off academy grounds. About 200 cadets in summer academics were being moved to the University of Colorado-Colorado Springs, and 350 others in airmanship and other training programs were released to local sponsor families, the school said. The cadet area isn't immediately threatened, and an incoming class of more than 1,000 is still scheduled to arrive Thursday.

About 120 soldiers from nearby Fort Carson built firebreaks around parts of the academy, aided by equipment including 10 heavy bulldozers, four excavators, 13 military transportation and support vehicles, and one commercial road grader, Army officials said.

Scope of damage unknown

The full scope of the fire remained unknown. So intense were the flames and so thick the smoke that rescue workers weren't able to tell residents which structures were destroyed and which ones were still standing. Steve Cox, a spokesman for Mayor Steve Bach, reported that at least dozens of homes had been consumed.

The FBI said it was investigating the cause of the blaze.

In addition to the some 30,000 evacuees, about 3,000 more people were evacuated to the west of the fire, Teller County authorities said Wednesday, and Teller County courts were closed through Thursday.

The Red Cross was accommodating victims at its shelters, with space enough for perhaps 2,500 people. Most evacuees were staying with family and friends.

Crews also were battling a deadly and destructive wildfire in northern Colorado and another that flared Tuesday night near Boulder.

"With over 10,000 firefighters in the Forest Service and the ability to get over 700 aircraft of all types, we're feeling cautiously confident when you look at the season as a whole," Harbour said.