IDAHO CITY, Idaho (AP) — 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.

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DAHO CITY, Idaho (AP) — A southwest Idaho wildfire burning timber in rugged terrain has reached a state highway and fire officials say they plan to use aircraft to full capacity on Thursday to try to prevent flames from jumping across.

Officials say the 16-square-mile blaze burning about 8 miles north of Idaho City reached State Highway 21 near a campground on Wednesday evening.

About 23 miles of the highway are closed from north of Idaho City to south of Lowman as firefighters remove trees and brush to reduce the potential for the fire to cross the road.

Officials say temperatures in the 90s and unstable air conditions have contributed to the fire's growth.

About 1,000 firefighters supported by 10 helicopters and retardant drops by other aircraft are fighting the blaze.

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IDAHO CITY, Idaho (AP) — Attempts by 900 firefighters backed by aircraft to halt the spread of a wildfire burning in rugged terrain in southwest Idaho have been unsuccessful with the blaze doubling to 14 square miles in size on Wednesday.

Officials say they've expanded a closure in the Boise National Forest and also started contingency plans to use State Highway 21 as a firebreak. About 23 miles of the highway are closed from north of Idaho City to south of Lowman as firefighters remove trees and brush to reduce the potential for the fire to cross the road.

Managers gave no timeline for when the well-traveled route from southwest Idaho to vacation areas in central Idaho will reopen. Officials say 10 structures are threatened and 35 people have been evacuated.