The Horton Fire, burning along the Mogollon Rim northeast of Payson, grew to nearly 900 acres and was 15% contained, as of Monday evening.

The Horton Fire, which covers 898 acres of ponderosa pine, conifer and chaparral, had not moved below the Highline Trail, a 57-mile route popular for backpacking, camping and hiking. The fire was holding along Forest Road 300 to Forest Road 9305.

Surrounding developments Tonto Creek Estates, Tonto Christian Camp, Tonto Fish Hatchery and Zane Grey Cabins remain in "set" status, meaning residents should be alert and prepared to leave, while 181 fire personnel fight the flames.

Smoke may be visible in the area for days, and the U.S. Forest Service urged people nearby with heart or respiratory issues to take precautions.

It is unusual to have wildfires during December.

Humans started the Horton Fire last week, but weather conditions have fueled it. The Arizona High Country has seen little precipitation over the past few months and warmer-than-average temperatures.

More: Arizona wildfires in December are 'definitely not normal.' Here's why there are still some

The Soap Fire, a human-caused fire that began last week a few miles west of Black Canyon City, is now 90% contained, according to the federal InciWeb website.

Both fires affect areas with steep, rugged terrain.

On Monday, fire crews working on the Horton Fire planned to clean up around recreational infrastructure on the north end of the fire. That included removing leaf litter and downed materials from kiosks, restrooms and signs.

While light precipitation was forecast in the middle of the week, it was not expected to bring significant rains.