Firefighters have been ambushed by sniper fire while responding to a blaze in a mountain community in the US state of Idaho, killing at least two people and unleashing barrages of gunfire over several hours in an attack the governor called a “heinous” assault.

The Kootenai County Sheriff’s Office said crews responded to a fire at Canfield Mountain just north of Coeur d’Alene around 1.30pm (6.30am AEST) and gunshots were reported about a half hour later.

Sheriff Bob Norris said officials believe the two people killed were fire personnel.

He didn’t know if anyone else was shot.

“We don’t know how many suspects are up there, and we don’t know how many casualties there are,” Norris told reporters at a news conference.

“We are actively taking sniper fire as we speak.”

People are still coming off the mountain, the sheriff said, so it “would be safe to assume” that others were still up there.

Governor Brad Little said “multiple” firefighters were attacked.

“This is a heinous direct assault on our brave firefighters,” Little said on the social platform X.

“I ask all Idahoans to pray for them and their families as we wait to learn more.”

Norris said it appeared the sniper was hiding in the rugged terrain and using a high-powered rifle.

He said he instructed deputies to fire back.

“I’m hoping that somebody has a clear shot and is able to neutralise, because they’re not at this point in time showing any evidence of wanting to surrender,” the sheriff said.

An alert by the Kootenai County Emergency Management Office asked people to avoid the area around Canfield Mountain Trailhead and Nettleton Gulch Road, about 6km north of downtown Coeur d’Alene.

The fire was still active, Norris said.

“It’s going to keep burning,” he said.

“Can’t put any resources on it right now.”

Coeur d’Alene is a city of 55,000 residents near the border with Washington.

Canfield Mountain is a popular hiking and biking area on the outskirts of the northern Idaho city, covered with trees and heavy brush and crisscrossed with trails.