Two firefighters have been killed and another seriously injured after they were ambushed by a sniper in the US north-west.

The firefighters were shot at as they responded to the blaze at Canfield Mountain, near Coeur d’Alene in Idaho’s north, about 1.30pm on Sunday (local time).

Barrages of gunfire lasted for hours, in an attack described by the state governor as “heinous”.

The Kootenai County Sheriff’s Office said later a tactical response team had found a man’s body in the area late on Sunday. Officials did not release his name, nor did they say what kind of firearm was found near the body.

A shelter-in-place order has since been lifted, but the fire is still burning.

“Residents in the area are advised to be prepared and ready should further action need to be taken,” the office said.

Earlier, Sheriff Bob Norris said he didn’t yet know if anyone else was shot and it wasn’t immediately clear if there was one gunman or more. He urged people to stay away from the area.

“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.”

The Canfield Mountain nature area, which is popular with hikers, is near Coeur d’Alene, a city of 57,000 people about 420 kilometres east of Seattle.

Governor Brad Little said “multiple” firefighters had been attacked.

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

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

Kim Anderson, a spokesperson for the local Kootenai Health Hospital in Coeur d’ Alene said three people involved in the shooting had been brought in. Two were dead on arrival while a third was being treated.

The International Association of Fire Fighters, a union that represents firefighters, said one person was undergoing surgery.

“While responding to a fire earlier today in Coeur d’Alene, Idaho, IAFF members were ambushed in a heinous act of violence,” it wrote in a post on X.

“Two of our brothers were killed by a sniper, and a third brother remains in surgery. Please keep them, their families, and law enforcement in your prayers.”

Fire authorities said the fire on the mountain was “gonna keep burning”.

“We won’t put any resources out there until the threat is neutralised,” Coeur d’Alene Deputy Fire Chief Bill Deruyter said earlier.

He said the fire danger was “at a high state right now”.

“It’ll calm down as the evening progresses but then it’ll pick back up tomorrow,” he said.

Norris said the shooter had used high-powered sporting rifles to fire rapidly at first responders, with law enforcement initially unsure of the number of perpetrators involved.

They were in a spot “with heavy brush and they are well prepared and blending in with their surroundings”, Norris 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 6.5 kilometres north of downtown Coeur d’Alene.

Law enforcement is investigating whether the fire could have been intentionally set in order to lure first responders to the scene.

“FBI technical teams and tactical assets are currently on the scene providing support,” FBI deputy director Dan Bongino wrote on X.

“It remains an active, and very dangerous scene.”

Gun ownership is widespread in the US, where the country’s constitution protects the rights of Americans to “keep and bear arms”.

Deaths related to gun violence are common – 17,927 people were murdered by a gun in 2023 in the US, according to the most recent available data from the Centres for Disease Control and Prevention.

-with AAP