U.S. firefighters on Saturday battled to gain control of a fast-moving wildfire raging on the edge of Yosemite National Park, threatening power and water supplies to San Francisco, about 200 miles to the west.

The so-called “Rim Fire,” which has grown to some 125,000 acres, remained largely unchecked. "We are making progress, but unfortunately the steep terrain definitely has posed a major challenge," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection. He described the fire as the 16th largest on record in California.

California Governor Jerry Brown declared a state of emergency Friday, warning that the fire had damaged the electrical infrastructure serving San Francisco and forced the city’s public utilities commission to shut down power lines. There have been no reports of blackouts.

A spokeswoman for the U.S. Forest Service said high humidity and firefighting efforts through the night had helped. "Firefighters are still working with the same difficult situation, and they're really taking every opportunity they can to take hold of this fire," she said.

The Hetch Hetchy Reservoir provides water to 2.6 million customers in the San Francisco area and Brown said the city's water supply could be affected. The fire remained about 4 miles west of the reservoir and more than 20 miles from Yosemite Valley.