The agency — created earlier this year to streamline firefighting and fire reduction across public lands — said the firefighters had been part of an interagency response to the Knowles and Gore fires on Saturday.
"The US Wildland Fire Service stands united with the USDA Forest Service in grief and in our unwavering support for the loved ones left behind," it said in a statement on Facebook on Sunday.
"Their bravery, dedication, and sacrifice will never be forgotten."
Wildfires are raging 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 in rugged terrain in southwest Utah.
It ballooned on Saturday to nearly 400sq/km after marching through canyons and mountainsides, destroying part of a ski resort and other summer cabins along the way.
Authorities in Beaver County began working with fire teams on Saturday to assess the extent of the damage, but no estimates were immediately available.
The cliffs and steep slopes have made the job even harder, said Alyssa Mason, a spokesperson assigned to the fire.
"It's hard to get dozers and other heavy equipment into that. It's hard to get engines into that," she said.
"It doesn't make it impossible to firefight, but it does just kind of slow things down."
Hundreds of firefighters have been arriving in the arid state 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.
The danger is even higher this year because of Utah's record-low snowpack and its warmest winter on record.
Much of the West is grappling with similar conditions, according to the National Interagency Fire Center.
From Alaska to Florida, crews worked on Saturday to corral dozens of fires, including three dozen that were classified as large and uncontained.
Nationally, 1.2 million hectares have burned since the start of the year. That is more than the 10-year average.
The conditions in Utah were critical enough for Governor Spencer Cox to declare an emergency earlier this week and clear the way for the state to ban fireworks ahead of the July Fourth holiday.
The order comes as Utah is experiencing one of the most severe wildfire seasons in recent history, fuelled by historic drought conditions.
State officials said that over the past week, Utah has seen an increase in wildfire starts, with each fire showing unprecedented behaviour. These starts have stretched the state's wildland firefighting capabilities, State Forester Jamie Barnes said.
Forecasters with the National Weather Service over recent days have been issuing red flag warnings for a wide swathe of the West, from California to Arizona and New Mexico.
South of Grand Canyon National Park, authorities said the flames of a new wildfire were moving away from Grand Canyon Village and the nearby community of Tusayan on Saturday.
But about 80km away, another fire prompted Coconino County officials to issue evacuation orders for those near Kendrick Mountain.
Parts of northern Arizona were without power Saturday as the utility serving the area initiated a safety shut-off in hopes of lessening the wildfire risk.
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