SAN FRANCISCO, California: The wildfire threatening Yosemite National Park has spread further into the US tourist landmark despite efforts by over 4,000 firefighters using planes and bulldozers, officials said Wednesday.
Ash is gathering on the surface of a reservoir serving San Francisco, but officials said water quality has not been affected.
The so-called Rim Fire -- California's seventh biggest ever -- now covers some 293 square miles (760 sq km), an area bigger than Chicago, according to the California Department of Forestry and Fire Protection (Cal Fire.
"Rapid fire growth and extreme fire behavior continue to hamper suppression efforts," firefighters warned in the latest update on the Inciweb multi-agency website.
The blaze, which broke out August 17, is now 23 percent contained, compared to 20 percent on Tuesday, it said. A total of 4,191 firefighters are now involved in battling it.
Highway 120, a main road approaching Yosemite from the west, remains closed.
From noon (1900 GMT) Wednesday, only two roads allowed access to Yosemite Valley, the spectacular area where most tourists visit, the park said on its website, announcing the closure of the stretch of the 120 Highway inside Yosemite.
On the eastern edge of the blaze, flames are racing unimpeded because the terrain is relatively flat.
"They're in scouting mode," Dick Fleishman of the US Forest Service told the Los Angeles Times, referring to fire crews. "There's not a lot of real good areas to get out in there and do a lot of work."
On Tuesday, firefighters used bulldozers to clear brush and vegetation from land to deny the fire fuel as it approaches the Tuolumne River, hoping it will not cross the waterway.
The blaze has forced the closure of multiple roads, campgrounds and other facilities in the area, and has also threatened a number of groves of giant sequoia trees, some of the world's biggest and oldest living organisms.
It remains more than 15 miles away from the majestic Yosemite Valley, visited by millions of tourists every year to see natural wonders including the Half Dome and El Capitan rock formations.
But ash from the inferno has reached the reservoir that supplies San Francisco's drinking water.
The Hetch Hetchy Reservoir is the main source of fresh water for 2.6 million people living in the San Francisco Bay Area, some 200 miles (320 kilometers) to the west.
No injuries or deaths have been reported due to the blaze, but it has destroyed at least 111 structures -- 31 homes as well as buildings on camp grounds that were hastily evacuated last week when the fire erupted.
More than 5,500 buildings, including 4,500 homes, remain under threat, according to Cal Fire.
The fire's potential to grow and the difficulty of the terrain were both still described as "extreme" by the Inciweb update.
But the park stressed again Wednesday that "most of Yosemite National Park is not affected by the fire and is relatively smoke-free. The northern part of the park... has some smoke. Conditions may change if winds shift."