A raging California wildfire burning on the northern tip of the storied Yosemite National Park now threatens San Francisco's water and electricity supply, officials say.
The Rim Fire has charred through 54,220 hectares, growing nearly 3399 acres overnight, according to InciWeb, the online Incident Information System that monitors fires in the western United States.
The fire was just seven per cent contained, InciWeb said, adding that it "remained fairly active overnight in most all divisions," and exhibited "rapid rates of spread, torching and spotting" on its eastern edge.
More than 2,800 firefighters, supported by helicopters and air tankers, are struggling to contain the blaze, which started on August 17 from still unknown causes.
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 - which lies some 320 kilometres to the west - because the Pacific coast city gets much of its electricity from the region affected by the fire.
The state's firefighting efforts got a boost when California received federal assistance over the weekend to help mobilise the necessary resources.
The San Francisco water and power utility said the city has not so far seen any interruptions in service, though two hydroelectric plants have sustained damage in the fire.
Crews were working on repairing one of the plants, the utility said on its website, and supplemental power supplies in the interim have cost some US$600,000.
The utility said it also had large reserves of water stored locally, which it will make available if the fire interrupts water delivery.
The fire broke out at Stanislaus National Park, which along with Yosemite is among the state's main natural tourist attractions.
Officials had ordered Tuolumne City and surrounding areas evacuated. Officials also closed a major interstate highway running through the region.
Brown has also declared a state of emergency in Tuolumne County, allowing him to use additional resources to battle the fire.
Satellite photos earlier showed giant columns of white smoke from the fire drifting far into the neighboring state of Nevada.
The Rim Fire "is expected to continue to exhibit very large fire growth due to extremely dry fuels and inaccessible terrain," Inciweb said.
It said fire crews were using direct attack tactics as well as indirect attacks, which create control lines away from the fire's active edge.