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(CNN) -- A sprawling wildfire in northern Colorado grew larger than the nearby city of Fort Collins on Monday, spewing towering plumes of smoke into the air and forcing thousands of people in its path to flee their homes.

The Red Cross, Humane Society and other aid groups mobilized to help evacuees while at least 400 firefighters, aided by air tankers and helicopters from as far away as Canada battled the fire about 15 miles west of Fort Collins.

"Yesterday when the winds had shifted to northerly winds we could see this massive, most amazing smoke cloud that looked almost like a volcanic eruption," said Grant Campbell, a Laporte, Colorado resident who is staying in his home despite an evacuation order issued Sunday.

"It really is beautiful in a kind of surrealistic way, but you also have to remember that it puts people and their homes in danger," he said.

The fire nearly doubled in size overnight to 36,930 acres, or 57 square miles, authorities said Monday. It had been estimated at 20,000 acres Sunday night. Fort Collins is 47 square miles.

While Fort Collins was not immediately threatened by the fire, a smoky pall hung over the city of 143,000, said Stephanie Ashley, a spokeswoman for the Larimer Humane Society.

"It's pretty much a haze covering the town, and you can definitely smell it," said Ashley, whose shelter was housing 170 animals dropped off by evacuees.

Although temperatures were slightly cooler and winds a bit less gusty than on Sunday, the fire is highly likely to continue growing, the incident management team reported Monday.

Paramedics treated one firefighter for heat exhaustion on Sunday, according to Larimer County authorities. No other injuries had been reported, although one person was listed as missing. Authorities were also looking into a report that two hikers were missing near the fire.

Crews worked around the clock to evacuate residents, in some cases as "flames were licking at the units that were doing the evacuations," Sheriff Justin Smith said Sunday.

"We have planned and trained for fires in every neighborhood. But this fire hit every neighborhood at once," Smith told reporters.

Although the exact number of people under evacuation orders was not known Sunday, authorities had put out more than 2,600 calls notifying residents of the evacuation orders, authorities said.

At least 18 structures, some of them homes, had been destroyed, authorities said.

"We know this number will increase once crews get their surveys done," officials said in a posting on the Larimer County Emergency Information website.

Hundreds more homes are threatened, Smith said Sunday. Some structures likely won't survive, he said.

Fueled by strong, gusting winds, low humidity, high temperatures and dry brush, the fire was behaving erratically, Smith said Sunday. It was burning in multiple directions and, in some cases, returning to scorch areas it had already burned, he said.

A shortage of equipment and manpower was compounding the problem. Regional resources are already fighting other outbreaks, such as the massive Whitewater Baldy fire in New Mexico.

A Type 1 incident response team -- the most advanced and capable available for a wildfire -- began working the Colorado fire Sunday night, according to Larimer County authorities. Additional crews had been ordered, authorities said.

Gov. John Hickenlooper ordered the Colorado Army National Guard into the fire fight Sunday, Guard spokesman Capt. Darin Overstreet said.

The National Guard's Black Hawk helicopters, equipped with water buckets, joined an increasing number of firefighting helicopters and air tankers being brought into the state to battle the fire. Air tankers from Canada arrived Sunday as well, officials said.

Poudre Fire Authority Chief Tom Demint told reporters Sunday night that additional aid is critical. Dozens of fire engines deployed can only do so much in corralling the charging blaze, he said.

"You do the math and you see what kind of (trouble) we're in," Demint said.

The cause of the fire is under investigation, but authorities believe lighting is to blame.

First measured at two acres early Saturday, the High Park fire has grown exponentially in the time since -- including more than doubling in size Sunday and again overnight into Monday.

CNN's Greg Morrison and Chelsea J. Carter contributed to this report.