More than 2,000 firefighters with the support of tanker planes and helicopters were combating Saturday a California forest fire that has covered more than 51,000 hectares (200 sq. miles) and threatens thousands of homes and other buildings.

Authorities say that San Francisco's water supply could go murky in the coming hours as it mixes with ashes from the wildfire near Yosemite National Park, some 310 kilometers (195 miles) to the east in an area that is part of the city's hydroelectric system.

The so-called "Rim Fire" swept into Yosemite National Park on Friday and on Saturday firefighters had controlled only 5 percent of the front of the blaze being spread by winds of up to 20 kilometers (12 miles) per hour.

Forest fires are part of a forest's natural cycle of maturation and renovation, but the expansion of human populations into previously uninhabited regions often requires firefighters' efforts, and at times the loss of their lives, to contain and extinguish them.

The flames have spread northeast to some 4 kilometers (2 1/2 miles) from the Hetch Hetchy Reservoir, whose dam and turbines produce almost 85 percent of San Francisco's electricity.

California Gov. Jerry Brown extended the state of emergency to the city and county of San Francisco due to the fire's threat to public services.

According to the authorities, the fire began seven days ago in Stanislaus National Forest, which, like Yosemite, is one of the state's main tourist attractions.

Some 4,500 homes and other buildings are endangered by the blaze, against which more than 2,000 firefighters are working with the support of tanker planes and helicopters that drop water and chemical compounds on the flames to slow the combustion. EFE