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.

Governor Polis declared a state of emergency for Colorado on Saturday, authorising the state's National Guard to join the response efforts.

"I'm devastated about the loss of three heroic firefighters who died in the line of duty in Western Colorado," he said in a statement.

"To the loved ones of those lost, and to their fellow crew members - some who are still battling the flames – know that the State of Colorado mourns alongside you," the governor added.

In Utah, the largest of eleven active blazes is the Cottonwood Fire, which as of Sunday covered over 93,000 acres and was 0% contained, according to the US Forest Service. Utah Governor Spencer Cox has also issued an emergency order banning the use of fireworks ahead of the 4 July weekend.

The fires spread across Utah and Colorado are in mostly sparsely-populated areas, but have threatened some resorts and cabins. Warmer winters and high winds have fuelled recent fires across much of the western US.

Climate change is making weather conditions that lead to wildfires, such as heat and drought, more likely, scientists say.

They predict wildfires will become more frequent and intense in future because of the combined effects of land use and climate change.