SAN FRANCISCO, California (AFP) - Firefighters reported progress Monday battling a huge blaze on the edge of Yosemite National Park, but warned it remains an "extreme" threat as it nears the top US tourist destination and San Francisco's water supply.

The Rim Fire, which began nine days ago, has grown to become the 13th largest in California's recorded history and has sparked the closure of one of the main roads into the spectacular natural beauty spot.

The fire has already crossed into the world-famous park, but it remains at least 15 miles from the Yosemite Valley, an area visited by millions of tourists each year looking to see stunning scenery, waterfalls and rock formations including Half Dome and El Capitan.

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

Local news media reported on Monday that flames from the Rim Fire were a mere five miles away from the reservoir.

The fire "is expected to continue to exhibit very large fire growth due to extremely dry fuels, strong winds and inaccessible terrain," according to the latest update on the Inciweb firefighting information website.

"Rapid fire growth and extreme fire behavior (are) hampering suppression efforts," it added.

The blaze has scorched 60,614ha and is 15 per cent contained, up from 7 per cent on Sunday, said Cal Fire, the California Department of Forestry and Fire Protection.

Some 4,500 buildings remain threatened by the fire, which is being tackled by nearly 3,700 firefighters. Schools in several nearby areas remained closed on Monday.

"Winds again today will pose a challenge with gusts of 42kmh out of the south, pushing the fire further to the north-east," said the official update.

Meanwhile, the park said in an update on its website, "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, remained closed on Monday due to the fire.

The San Francisco Public Utilities Commission said there have been no interruptions in the city's water supply, and that there has been no change in the quality of drinking water.

It added that it has large reserves of water stored locally, which it will make available if the fire interrupts delivery.

However, the city also relies on hydroelectric power from the same region: two of its three power plants were shut down when the fire swept through, Tyrone Jue of the Utilities Commission told the San Francisco Chronicle San Francisco kept the power flowing by relying on agreements with other utility companies and buying supplemental power.

One of the plants was still too dangerous to reach on Sunday, but repairs on the other were underway and should be completed by Monday, Jue told the paper.

Extremely dry conditions due to a prolonged drought, coupled with inaccessible terrain in the affected area, have exacerbated the severity of the incident.

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

The state's firefighting efforts got a boost when California received federal assistance over the weekend to help mobilise the necessary resources.

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," the White House said.

"The President reiterated his commitment to providing needed federal resources to support the ongoing state and local response," it added in a read-out of the call.