SAN FRANCISCO  - The wildfire threatening Yosemite National Park is still burning its way into the huge US tourist attraction despite dogged efforts by thousands of firefighters using planes and bulldozers.

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 281 square miles (731 sq km) , an area bigger than Chicago.

The fire, which broke out August 17, was still only 20 percent contained compared to 15 percent on Monday.

Highway 120, a main road into Yosemite from the west, remains closed as more than 3,700 firefighters battle the flames. 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 area at the heart of the park, 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.

The fire is also threatening the iconic giant sequoia trees in Yosemite.

“They are definitely in danger, but we’re doing everything we can,” US Forest Service spokesman Lee Bentley told CNN.

Bentley said firefighters had “a pretty good chance of keeping it away... but it’s going to take a heck of a lot of work and a lot of air power.”

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.

“Damage assessments from the Berkeley Tuolumne Camp revealed extensive losses of infrastructure. Rapid fire growth and extreme fire behavior are hampering suppression efforts,” said the latest update by the multi-agency Inciweb website.

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 Tuesday 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.”