Thousands of homes remain under threat as California’s biggest wildfire continues to gain strength across western parts of the US.

It comes amid fresh calls for politicians in Washington to take “bold action” against climate change as more than 85 large wildfires continue to burn around the country.

The Dixie Fire started on 14 July and has already destroyed more than a dozen houses and other structures.

It has combined with smaller fires and has been sweeping through the small north Californian community of Indian Falls.

Rick Carhart, spokesman for the California Department of Forestry and Fire Protection, revealed that firefighters carrying hand tools were forced to hike through rugged terrain where emergency vehicles cannot reach.

He said: “It has been burning in extremely steep canyons, some places where it is almost impossible for human beings to set foot on the ground to get in there.

“It’s going to be a long haul.”

Fire officials said the blaze had charred nearly 298 square miles of timber and brush in Plumas and Butte counties and is now estimated to be 21% contained.

The fire has led to evacuation orders in several small mountain communities and along the west shore of Lake Almanor, a popular resort area in north California where about 10,000 homes remain under threat.

Firefighters also reported progress against the nation’s largest wildfire, the Bootleg Fire in southern Oregon, containing 46% of the blaze that had consumed nearly 640 square miles.

More than 2,200 firefighters battled the blaze and had worked on constructing containment lines at the north and eastern edges to try to hold back the flames.

The fire, which is said to have been caused by lightning, has burned 67 homes, mainly cabins, and at least 2,000 houses were under evacuation orders.

Oregon governor Kate Brown told CNN’s State of the Union that it is imperative federal and state authorities invest in mitigation measures such as tree thinning to limit the number of similar massive blazes.

But she conceded that “the harsh reality is that we’re going to see more of these wildfires”.

On Twitter, she added: “We are feeling the impacts of climate change in real time here in Oregon.

“Historic fires, extensive drought, unprecedented heat – we need bold action from Congress to complement the steps we’re taking at the state level.”

The fires in the US are mostly in western states.

Elsewhere in California, the Tamarack fire south of Lake Tahoe continues to threaten communities on both sides of the California-Nevada state line.

In Montana, officials were focusing on structure protection for three fires amid weather forecasts of rising temperatures, low humidity and westerly winds.

In Washington state, firefighters battled two blazes in Okanogan County that threatened hundreds of homes and caused hazardous air quality conditions.