By Laura Zuckerman
SALMON, Idaho (Reuters) - Sixteen firefighters in Idaho held their own on Tuesday against a fierce wildfire that forced dozens of their colleagues to retreat as it raged along the Montana border, devouring 18,000 acres of rugged forest.
The 16 crew members hunkered down around a small ranch in the Franck Church River of No Return Wilderness after 40 other firefighters were pulled out of the area on Monday as the blaze grew in size and intensity, the U.S. Forest Service said.
Agency officials said the 16 firefighters were not trapped and could be reached by helicopter airlift if necessary.
Jesse Bender, a spokeswoman for the Salmon-Challis National Forest, said the crew was left behind to defend ranch structures by clearing dense vegetation that could serve as potential fuel and laying hose lines for sprinkler grids.
The blaze grew from two lightning-caused fires that erupted August 10 and 14 in east-central Idaho, about 35 miles northwest of Salmon, then merged on Monday and spread into the Bitterroot National Forest in western Montana.
A tall mushroom cloud from the blaze, churning through pine, spruce and fir forest that has not burned for a century, was visible for over 100 miles.
The combined conflagration rapidly grew from about 700 acres early on Monday to 18,275 acres as of Tuesday morning, ranking it as the largest active wildfire in the nation, according to the National Interagency Fire Center in Boise.
About half of the burned acreage lies in Idaho, the other half in Montana, where fire crews were girding to protect homes west of the towns of Darby and Sula.
No injuries have been reported, but the blaze has burned through a popular campground and hot springs northwest of Salmon, Idaho, and threatened a lookout station in the area.
To the south, firefighters managed to contain a separate blaze by Monday night that had burned over 100 acres west of a federal nuclear reactor complex at the Idaho National Laboratory.
That fire started from a vehicle with a blown tire dragging the metal rim along the pavement of a state highway near the laboratory. Officials said there was no known radiation threat to the public.
(Writing by Steve Gorman; Editing by Cynthia Johnston)