Firefighters in California are making slow progress against a wildfire that has burned nearly 650 square km of forest near Yosemite National Park. The Rim Fire is now 20% contained, but it continues to spread.
The fire is at the edge of the Hetch Hetchy reservoir, which supplies San Francisco with 85% of its water.
The BBC reports visibility in the area is down to 30 metres.
The blaze began on 17 August in the Stanislaus National Forest.
Governor Jerry Brown has declared a state of emergency for San Francisco.
The BBC reports the blaze is also threatening power lines that carry electricity to the city.
Two hydroelectric power stations serving the city have been shut down, forcing the San Francisco Public Utilities Commission to spend $US600,000 buying power on the open market.