LARIMER COUNTY, Colo. -

A single-engine air tanker is helping firefighters as they try to contain a wildfire burning about 20 miles northwest of Fort Collins.

The U.S. Forest Service says the fire was burning through timber, grass and brush Monday on an estimated 250 to 280 acres northeast of the Hewlett Gulch Trailhead in Poudre Canyon.

Two trails in the area are closed.

No structures were immediately threatened, but some residents were given pre-evacuation notices to prepare to leave.

The fire is within a mile of Poudre Park.

The Forest Service says a heavy air tanker and helicopter also were requested to help fight the fire from above.

The cause of the fire wasn't immediately known.