The lightning-caused Indian Ridge Fire, burning 30 miles southwest of Darby in the Selway Bitterroot Wilderness, has grown 4,000 acres, according to InciWeb.
That's an increase of 1,438 acres from the last time the fire was mapped.
Officials report the fire has been active along the western and northern flanks, north of Indian Creek.
The fire is burning in a steep, rugged remote terrain where surface fuels and dead timber are making it difficult for firefighters to access the area.
Fire managers expect the blaze to reach Paradise Road and Indian Creek Campground in the coming days.
Temporary road closures including a barricade and road guard at Magruder Crossing could be put in place later this week.
The wildfire was first reported on July 7.