Officials in California say a fast-moving wildfire could threaten the water and electricity supplies to millions of people in the San Francisco area.
Known as the Rim Fier, it covers nearly 500 square kilometres and is threatening at least 5000 homes.
The BBC reports the fire is now 5% contained, officials say, up from 2% on Friday.
More than 2,700 firefighters are tackling the flames in difficult terrain.
Evacuations, some voluntary and some mandatory, are taking place. Despite the threat to thousands of homes, only a few have been destroyed.
California Governor Jerry Brown has declared a state of emergency for San Francisco about 240 kilometres away as the blaze is threatening power lines that bring electricity to the city.
It is burning within six kilometres of the Hetch Hetchy reservoir - where the city gets 85% of its water and some of its power.
Two of the three hydroelectric power stations serving the city were shut down.
The blaze, which continued for a week on the edges of Yosemite National Park, reached the park's backcountry at Lake Eleanor on Friday. But it remains some 32 kilometres away from Yosemite's main tourist area.