Utah authorities say a rural wildfire near the Idaho border has torched about 30 square miles and destroyed three structures.

Box Elder County Fire Marshal Corey Barton said Friday the fire has burned a ranchers' cabin as well as a camp trailer and a mobile home.

All three were vacant, and no one has been hurt. A handful of isolated farmhouses and other structures in the mountainous area are threatened.

Barton says investigators believe someone accidentally started the blaze, but he couldn't immediately say exactly what sparked it.

Fire authorities say smoke has drifted into Pocatello Valley in Idaho and the Salt Lake City area, about 85 miles south of the fire.

Firefighters have contained approximately 30 percent of the fire west of Plymouth with the help of aircraft and bulldozers.