SAN FRANCISCO, California, Aug 27, 201 (AFP) - The wildfire threatening Yosemite National Park grew on Tuesday to become the seventh biggest ever in California, as firefighters battled to keep ash from contaminating San Francisco's main water supply.

The so-called Rim Fire now covers 72,633ha, taking it into California's top 10 biggest ever blazes from 13th on Monday.

By Tuesday it was 20 per cent contained, up from 15 per cent the previous day.

A main road into Yosemite, one of America's top tourist destinations, remains closed as more than 3,700 firefighters battle the flames.

"I see another intense fire day," said Mr Lee Bentley of the United States Forest Service.

"Fire activity remained fairly active overnight, with active surface fire and torching and spotting," said the latest update on the Inciweb multi-agency website, which gives details of the firefighting effort.

The blaze has forced the closure of multiple roads, camp grounds and other facilities in the area, and has also threatened a number of groves of giant sequoia tree, some of the biggest and oldest living organisms on the planet.

But it remains more than 24.14km 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.

Although ash from the inferno has reached the reservoir that supplies San Francisco's drinking water, crews said they were confident public health can be protected, local news reports said.

The Hetch Hetchy Reservoir is the main source of fresh water for 2.6 million people living in the San Francisco Bay Area, located some 320km to the west.

"Water quality is not currently impacted by the Rim Fire," the San Francisco Public Utilities Commission said in the latest update on its website.

"We are monitoring water quality every minute of each day to ensure its quality and safety."

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

"They are definitely in danger, but we're doing everything we can," Mr Bentley told CNN when asked about the trees.

The Merced and Tuolumne groves "are not currently in imminent danger", according to Yosemite's park website.

Mr 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."

Schools in several nearby areas were expected to stay closed on Tuesday.

No injuries or deaths have been reported due to the blaze, but it has destroyed at least 23 structures - including on camp grounds that were hastily evacuated last week when the fire erupted - and threatens 4,500 more, officials say.

"Damage assessments from the Berkeley Tuolumne Camp revealed extensive losses of infrastructure. Rapid fire growth and extreme fire behaviour are hampering suppression efforts," said the latest Inciweb update.

But the park stressed on its website on 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."

Highway 120, one of the main routes into Yosemite from the west, was closed due to the fire.

Governor Jerry Brown has declared a state of emergency for San Francisco due to the threat to its water and even power supplies.

Two hydroelectric power plants have not been in use since Aug 19 due to the fire, the San Francisco Public Utilities Commission said on its website.

President Barack Obama spoke to California's governor on Sunday and "expressed his gratitude for the brave men and women working tirelessly to combat this devastating fire".