The Pioneer fire. a southwest Idaho wildfire burning timber in rugged terrain, on Thursday crossed a state highway fire officials had hoped to use as a firebreak.
Officials say the 20-square-mile blaze burning west to east crossed State Highway 21 about 5 miles south of Lowman.
About 23 miles of the highway are closed from north of Idaho City to south of Lowman.
Fire spokeswoman Susan Blake says ground crews and engines are responding to the area east of the highway where the fire crossed.
Officials say temperatures in the 90s and unstable air conditions have contributed to the fire's growth and that it's advancing by sending out embers ahead of flames.
About 1,000 firefighters supported by 10 helicopters and retardant drops by other aircraft are fighting the blaze.