A firefighter was killed on Saturday while battling a wildfire burning near Yosemite National Park, California state fire officials said, bringing to at least 10 the number of such deaths this year as wildfires rage across the US West.

Braden Varney was killed on the fireline while operating a bulldozer at the Ferguson Fire early this morning, said Scott Maclean, a spokesman for the California Department of Forestry and Fire Protection (CalFire).

Further details on Varney's death were not immediately available.

"The winds are really erratic, there's just no moisture in the vegetation, it is still so dry regardless of what rains we've had," Maclean said.

Varney was using the bulldozer to try to hold back a blaze that has burned 150 acres (61 hectares) in rugged, inaccessible forest near Savage Trading Post, some 20 miles (32 km) southwest of Yosemite in California's Sierra Nevada mountains and about 140 miles (225 km) east of San Francisco.

Nine U.S. wildland firefighters had been killed up to Tuesday as the United States faced another extreme year for wildfires, according to National Interagency Fire Center (NIFC) data.

Fourteen wildland firefighters died in all of 2017 when the United States suffered its second worst wildfire year on record, with 10 million acres (4 million hectares) blackened.

Varney worked for CalFire. The last CalFire firefighter to die was in December 2017 on the Thomas Fire, the largest in California's history and one of a rash of blazes that destroyed thousands of homes and killed 43 people in the state.

Nearly all of California faces abnormally dry or drought conditions, even though the state's historic 2012-2017 drought is officially over, according to the Drought Monitor agency.

The state has had its worst start to the fire period in a decade, with 220,421 acres burned through Thursday morning, according to NIFC data.

U.S. wildfires have already burned more than 3.3 million acres this year, more than the year-to-date average of about 3 million acres in the past 10 years, according to the NIFC tracking website.

As the summer continues, the risk of large wildfires is set to reach normal levels in much of the Southwest and Rocky Mountain areas, thanks to strong summer rains, but risk levels will remain above normal in California through at least October, according to NIFC data.