An out-of-control wildfire burning in steep terrain near Yosemite National Park has grown to more than 16,000 acres and is still threatening more than 2,000 houses, hotels and camp buildings, the Forest Service said Wednesday.

The blaze in the Stanislaus National Forest grew overnight to more than 25 square miles and was only 5 percent contained, Forest Service spokesman Jerry Snyder said.

The fire has forced the evacuation of at least two campgrounds and led a few dozen people to leave the tiny community of Buck Meadows, the area of Harden Flat and a stretch along Ferretti Road.

A number of camps have also been evacuated, including Camp Tawonga, San Jose Family Camp and Berkeley Camp. About 200 senior citizens and a few dozen staffers were also evacuated from Camp Mather, which is owned and operated by the city of San Francisco.

Meanwhile, San Francisco officials have powered down electrical lines running through the region from its Hetch Hetchy hydropower plant.

Officials told KTVU they took the measure over fears of power line arcing caused by airborne soot and smoke.

They added the blaze was not threatening the plant nor did it pose a threat to the water supply coming from the project.

The fire, which started on Saturday for unknown reasons, has already destroyed two residences and seven outbuildings, and about 2,500 structures were under threat in the community of Groveland, Snyder said.

State Route 120 was shut down in both directions for a 4-mile stretch, blocking traffic in and out of Yosemite on its west side, but the park remained open to visitors via state Routes 140 and 4.

About 850 firefighters were working the blaze, aided by aircraft.