A wildfire burning in the tinder-dry mountains 15 miles east of Ft. Collins, Colo., exploded in size today, growing from just a few acres to 5,000 acres-plus in a matter of hours.

"It was a tough day," Larimer County Sheriff Justin Smith said. "I've been with the agency for 21 years. This is probably the fire we were always concerned we might possibly have."

Hundreds of people have been evacuated and at least 10 structures have burned, although it's not known if they are houses. There are many homes in the fire zone and Smith said there are a unknown number of people unaccounted for.

"Some people felt fairly confident that there may have been people trapped back there, " Smith said. "I'm very concerned with some of the reports."

Two hundred firefighters are on the scene with more on the way. Five small air tankers, four helicopters and two heavy air tankers are already in the air with two more requested.

Much of Colorado has been under "red flag" fire weather warnings today, with temperatures higher than 90 degrees.

The blaze-dubbed the High Park Fire - has already been upgraded to the most serious level, a so-called "Type 1? incident, meaning a federal command staff will take over strategy for fighting the fire beginning Sunday.