The firefighters deployed emergency shelters for themselves during the so-called burnover - which occurs when a fire spreads and closes off all escape routes - on Saturday in Mesa County, Colorado, the US Interior Department said.
They worked for the US Wildland Fire Service and US Forest Service and were part of an interagency response to the Knowles and Gore fires, which merged with other fires to form the Snyder Fire. So far, about 100sq/km have burned.
The Wildland Fire Service, created earlier this year to streamline firefighting and fire reduction across public lands, said in a statement that it "stands united" with the Forest Service in grief and "in our unwavering support for the loved ones left behind".
"Their bravery, dedication, and sacrifice will never be forgotten," the statement said.
The names of the firefighters who perished were being withheld pending notifications to their loved ones, the Interior Department said.
Temperatures in Grand Junction - east of the fire - hit 34C with winds gusting to 70km/h, according to the National Weather Service.
The Mesa County Sheriff's Office was asking people to evacuate the potential path of the fire and to turn on irrigation water to saturate the land. The federal Bureau of Land Management closed public access to lands it manages nearby.
"Firefighter and public safety are the highest priority," the agency said in a statement.
"The temporary closing of the lands is to reduce exposure to hazardous situations due to the rapid rates of spread and fire behaviour that the fire has exhibited. The public is to remain clear of these closed lands".
Wildfire activity has intensified across the Western United States, as consecutive days of hot, dry and windy weather have fuelled flames in Utah, Arizona and elsewhere as new fires popped up across the region.
The largest blaze, the Cottonwood Fire, was burning out of control in rugged terrain in southwest Utah. It ballooned Sunday to more than 380sq/km after marching through canyons and mountainsides, destroying part of a ski resort and other summer cabins along the way. The cause is under investigation.
Firefighters were working on multiple fronts to slow the blaze, including using bulldozers to scrape away brush and trees to starve the fire of fuel it needs to burn.
Hundreds of firefighters have been arriving in Utah to battle new starts as well as those that have been growing because of what forecasters called critical fire weather - dangerously low humidity levels, warm temperatures and gusty winds.
12°C