Sierra wildfire now California's fourth-largest

The wildfire burning in and around Yosemite National Park has become the fourth-largest conflagration in modern California history, fire officials said as clouds and higher humidity helped crews further contain the biggest blaze in the United States this year.

The 2-week-old Rim Fire moved up a spot on the state's list of large wildfires dating back to 1932 when it grew to 901-square-kilometre - an area larger than the cities of San Francisco, Oakland and San Jose combined - on Saturday, California Department of Forestry and Fire Protection spokesman Daniel Berlant said.

Although the fire still is growing, it was 40 per cent contained as of Sunday, up from 35 per cent a day earlier.

Moister air was expected to slow flames from advancing through brush and trees, giving firefighters room to set backfires, dig containment lines and to strengthen lines around threatened communities, US Forest Service spokeswoman Pam Baltimore said.

Full containment is not expected until September 20.

The blaze started August 17 in the Stanislaus National Forest and two-thirds of the land burned since then is located there as well. In Yosemite, 94 square miles have burned. The cause remains under investigation, Baltimore said.

"Either way, if it was lightning or human-caused, they have not released any findings and we are not sure if and when that will be released," she said.

Meanwhile, the dense smoke that obscured Yosemite's majestic views for the first time on Saturday and prompted air quality warnings was starting to ease, park spokeswoman Kari Cobb said.

Although park officials advised visitors to avoid heavy exertion, Cobb said she has seen people outside running "and enjoying Yosemite, despite the smoke".

"The park was actually busier than I thought it would be," she said.

A 1105-square-kilometre fire in San Diego County that killed 14 people and destroyed more than 2,800 structures a decade ago tops the list of California's largest wildfires.

The Rim Fire has claimed 111 structures, 11 of them homes, but no lives.

- AP