LOS ANGELES, United States -- 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 neutralize the threat."

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

Serious gun violence is common across the United States, where many states have few barriers to the purchasing of firearms, despite widespread support for greater gun control.

According to the Gun Violence Archive, there have been 189 mass shootings in the United States this year -- defined by the organization as incidents where at least four people are killed or wounded, other than the shooter.

Agencies