Three firefighters died Wednesday in Washington state while battling a massive blaze.
More than 30,000 firefighters have been deployed in 10 US states, and the military also sent troops in the fight the fires.
President Barack Obama sent his condolences to the families of the deceased on Thursday.
The firefighters were responding to the so-called Twisp Fire in northern Washington when they were involved in a vehicle accident and a fire overtook their truck, media reported.
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Obama said their deaths were a "tragic loss," according to deputy spokesman Eric Schultz.
"On behalf of a grateful nation, the president's thoughts and prayers are with the families of these brave Americans," Schultz said in a statement.
Four other firefighters were also injured while responding to the blaze, but it was not immediately clear if they were involved in the same accident, according to CBS News.
Several wildfires were burning in Washington state, and officials urged nearby residents to evacuate as fires spread.
Washington Gov. Jay Inslee also sent his condolences after learning of the firefighters' deaths.
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"My heart breaks over the loss of life ... they gave their lives to protect others. It was their calling, but the loss for their families is immense and I know the community will come together to support them," Inslee said in a statement.
He warned that conditions in the region remained precarious and urged residents to evacuate if necessary.
"The conditions throughout the area remain extremely dangerous and I hope residents and visitors will heed evacuation orders or other emergency directions," he added.
The 10 states affected by large fires are Arizona, California, Idaho, Louisiana, Montana, Nevada, North Carolina, Oregon, Texas and Washington.