FORT COLLINS, Colo. (AP) -- Crews fighting a wildfire burning in Colorado are getting help from dozens more firefighters.

The fire is burning on an estimated 270 to 280 acres about 20 miles northwest of Fort Collins. Fire spokeswoman Reghan Cloudman says four, 20-person crews headed to the area Monday night to fight the fire Tuesday. A heavy air tanker and helicopter also have been requested.

No structures were immediately threatened, but Cloudman says calls were made to 160 phone numbers telling residents to prepare to leave. Cloudman says the fire is within a quarter mile of homes.

A single-engine air tanker helped fight the fire from the air after it broke out Monday in the Roosevelt National Forest.

The cause of the fire hasn't been determined.

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