The Mount Olympus and Aspen fires, both in rugged hard-to-reach areas, were thought to be only about an acre in size when spotted, The Salt Lake Tribune reported Wednesday.
Fire officials believe lightning sparked both fires, the report said. The state has been plagued by a drought that has left brush and forests highly susceptible to combustion.
Firefighters hiked into the area of the Aspen Fire east of Richmond late Tuesday and began trying to contain the blaze. They have not reported any injuries or any threats to structures.
The Mount Olympus Fire, located near the summit, is accessible only by foot or air.
Crews were still working to contain two other wildfires.
The Sinbad fire about 30 miles east of Moab has burned about 220 since lightning ignited it Sunday. It is about 5 percent contained, said incident commander Kevin Cahill.
The 180-acre Rock Creek Fire 15 miles east of East Carbon was technically contained, but firefighters remained on the scene to watch for flare-ups.