Three firefighters were killed and two were injured while fighting fires on the Colorado-Utah border, the U.S. Wildland Fire Service said.
The agency said the firefighters had been part of an interagency response to the Knowles and Gore fires.
The Department of the Interior said in a press release that the five firefighters were involved in a "burnover incident," in which crews are unable to find an escape route and must shelter as best they can while a fire passes over them, and that the two who survived were being treated for burn injuries.
Western Slope Fire officials said the five victims were locally based federal firefighters who deployed emergency shelters. Officials also said these were the first reported casualties in connection with Utah wildfires this season.
The Snyder Fire began as the Snyder Mesa Fire in east Utah's Grand County before crossing into Colorado and combining with the smaller Jones and Knowles fires in Mesa County to form the Snyder Fire, and officials with the Bureau of Land Management said it appears the fires may have merged into one incident while they await confirmation.
The Snyder Fire has burned an estimated 28,000 acres and was at 0% containment, prompting evacuation warnings for smaller communities in Mesa County.
The Cottonwood Fire in southwest Utah's Piute and Beaver counties has burned tens of thousands of acres, destroyed part of a ski resort and summer cabins, and has been reported as the largest wildfire burning anywhere in the United States.
Crews said days of low humidity, high temperatures and strong winds pushed fire behavior to extremes, with single-digit humidity readings, wind gusts near 45 mph and fuel moisture between 2% and 8%; those conditions briefly grounded helicopters and other firefighting aircraft.
Gov. Jared Polis declared a disaster emergency Saturday and authorized use of the National Guard to help fight the blazes, and Gov. Spencer Cox declared a state of emergency in Utah earlier in the week and posted that he and the first lady were praying for the crews and their families.
The U.S. Wildland Fire Service said its immediate focus was on supporting the firefighters' families, friends and crewmates, praised the crews' "bravery, dedication, and sacrifice" and said it would share more information when it is available to be released.
Five Grand Junction firefighters were airlifted to safety near Glade Park, and the sheriff issued pre-evacuation orders and closed roads in western Mesa County.
Federal officials said the extreme conditions have stretched resources, and the National Interagency Fire Center reported that nearly 3 million acres have burned across the country since the start of the year.