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

The blaze continued to rage on the mountainside in Kootenai County 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 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 believe the two people who were killed were firefighters.