Three firefighters were killed and two were injured while battling massive wildfires on the Colorado-Utah border on Saturday, officials say.

The firefighters were part of a federal response team assigned to the Knowles and Gore fires. They were suddenly overwhelmed by flames and tried to take shelter, the Department of the Interior said.

The two injured firefighters suffered burns and were taken to hospital for treatment.

"Our immediate focus is on supporting their families, friends, and fellow crewmates during this incredibly difficult time," the US Wildland Fire Service said of the firefighters, whose names have not been released.

The Knowles and Gore fires have since spread and merged with other fires to form the Snyder Mesa fire, which as of Saturday had burnt around 28,000 acres of land, Colorado Governor Jared Polis's office said.

The deaths and injuries happened during a "burnover incident", the US Wildland Fire Service said. That is when a fire spreads so suddenly and rapidly that it overtakes personnel or equipment, cutting off escape routes and forcing trapped firefighters to deploy an emergency, last-resort shelter tent.

"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," the service said. "Their bravery, dedication, and sacrifice will never be forgotten."

The five firefighters were from two federal agencies that oversee the management of public lands, the US Wildland Fire Service and the US Forest Service.

The wildland service, which is part of the US Department of the Interior, was established in January this year to streamline firefighting efforts across public lands.