BOISE - Wildfire season accelerated in Idaho on Thursday as evacuations were ordered in the northern part of the state and a massive fire straddling the Idaho-Oregon border grew to 340 square miles.

In southwest Idaho, giant swaths of grassland and sagebrush were consumed by the giant blaze that has intermittently closed U.S. Highway 95, which was reopened Thursday. Winds and temperatures around 100 degrees were expected.

"It's a challenging situation to try to contain a fire right now," fire spokeswoman Karen Miranda said.

At least 15 square miles of primary sage grouse habitat has also burned. Habitat will be a key consideration this fall when federal officials are expected to decide if the birds need protection under the Endangered Species Act.

Miranda said firefighters were trying to protect the old growth sagebrush, much of it located in areas administered by the U.S. Bureau of Land Management.

"It's the No. 1 natural resource priority for the BLM," Miranda said. "That habitat is being protected as well as we can without compromising firefighter safety and protection of private property."

About 215 firefighters backed by five helicopters, 16 fire engines and eight bulldozers were assigned to the blaze.

Mandatory evacuations were in place in a rural area near the town of Kamiah as a complex of 21 fires has burned about 6 square miles.

Fire spokeswoman Hailey Graf said it was not clear how many residents were under the mandatory evacuation order. She said no buildings have been lost.

"Firefighters are right on the fire line working to contain it as efficiently and effectively as possible," she said.

In central Idaho, crews were fighting a 500-acre fire burning in heavy timber about 13 miles north of Crouch. Officials said half of the fire is in Boise National Forest and half on Idaho endowment land. Crews were also battling several smaller fires in the forest.

In Boise, a wildfire Thursday afternoon threatened homes on the northern edge of the city before a firefighting effort that included aircraft dropping retardant beat it back. Officials closed 36th Street heading north out of the city.

Meanwhile, federal officials at the National Interagency Fire Center in Boise raised the National Fire Preparedness Level to its highest level due to increased fires across the West.

The decision means additional assistance from the military and international help can be requested.