A wildfire threatening Yosemite National Park is endangering San Francisco's power supply as more than 2600 firefighters struggle in rough terrain to tame the massive blaze.
Only five per cent of the so-called Rim Fire was contained, authorities said on Saturday, with the US Forest Service labeling the potential for further growth as "extreme" given the dry conditions and location in steep canyon areas.
The 518-square-kilometre fire is scorching the remote northwest edge of Yosemite.
However, the park, which is popular with tourists as a hiking and climbing destination, remains open.
The Forest Service says 4500 structures are in danger and evacuations for several nearby communities have been ordered. There have been no reported fatalities.
The out-of-control inferno is also threatening the power grid of San Francisco, some 300 kilometres to the west.
California Governor Jerry Brown declared a state of emergency for San Francisco late on Friday, saying transmission lines and other electricity infrastructure serving the city were vulnerable.
The cause of the fire, which began last week, is unknown.
