FIREFIGHTERS are battling to contain a fast-moving wildfire raging on the edge of Yosemite National Park.

The blaze is threatening power and water supplies to San Francisco about 200 miles to the west. The "Rim Fire", which covers about 125,000 acres, is largely unchecked with extreme terrain hampering efforts at containment.

There are fears San Francisco's water supply could be affected if the blaze reaches the Hetch Hetchy Reservoir. The fire is only a few miles west of the reservoir and about 20 miles from Yosemite Valley, the park's main tourist centre.