A shooter ambushed firefighters responding to a blaze in an Idaho mountain community on Sunday, 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 at about 1:30 p.m., and gunshots were reported about a half hour later.

Sheriff Bob Norris told reporters that officials 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 said. "We are actively taking fire sniper as we speak."

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

Idaho Gov. Brad Little said "multiple" firefighters were attacked.

"This is a heinous direct assault on our brave firefighters," Little said on the social media platform X. "I ask all Idahoans to pray for them and their families as we wait to learn more."

The sheriff's office in neighbouring Soshone County said on Facebook that authorities were "dealing with an active shooter situation where the shooter is still at large."

An alert by the Kootenai County Emergency Management Office asked people to avoid the area around Canfield Mountain Trailhead and Nettleton Gulch Road, about 6.5 kilometres 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."

No additional details were provided.

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