At least two people were killed Sunday when firefighters tackling a blaze were ambushed by an unknown number of gunmen in an ongoing standoff in the northwestern US state of Idaho, the local sheriff said.

A blaze was still raging on the mountainside in Kootenai County, a popular park and hiking area, as law enforcement exchanged gunfire with the shooter or shooters, Sheriff Robert Norris told reporters.

"We now have two deaths. We have an unknown amount of casualties. We still have civilians that are coming off of that mountain.... We are actively taking sniper fire as we speak," he said at a press conference.

The attack was carried out with high-powered rifles, the sheriff said, stressing that the goal of law enforcement was to track the assailant or assailants down and shoot on sight.

The shooter showed "no evidence of wanting to surrender," he continued, adding: "As soon as somebody has a clear shot, I encourage them to take that shot to neutralise the threat."

He said authorities believed the two people who were killed were firemen.

Norris said authorities did not believe the suspects were barricaded inside a building, but were in "heavy brush" and were "blending in with their surroundings."

They were "well-prepared," he added, saying authorities did not know if there was "one, two, three or four" shooters.

The deputy director of the FBI, Dan Bongino, described it as "an active scene" and said the bureau was sending personnel "to provide tactical and operational support."

Footage from the TV station and a photo published by NBC showed dense white smoke billowing above a hilly, forested area. (AFP)