In this undated photo provided by the U.S. Forest Service, the Rim Fire burns near Groveland Ranger Station in Groveland, Calif.
(CBS/AP)-- A wildfire burning on the northern border of Yosemite National Park grew more than 7 square miles overnight as firefighters have gained little ground in slowing it.
The fire is threatening thousands of rural homes, as well as San Francisco's water and energy supply.
Fire officials say crews are focused east of the North Fork of the Tuolumne River, and are struggling to keep the fire from encroaching more into the park.
Firefighters were bracing for strong south winds of up to 26 mph, so officials say the fire is likely to continuing growing.
The massive blaze is also burning in the vicinity of two groves of giant sequoias that are unique to the region, prompting park employees to take extra precautions of clearing brush and setting sprinklers.
The towering trees, which grow only on the western slopes of the Sierra Nevada and are among the largest and oldest living things on earth, can resist fire. However, dry conditions and heavy brush are forcing park officials to take extra precautions in the Tuolumne and Merced groves. About three dozen of the trees are affected.
"All of the plants and trees in Yosemite are important, but the giant sequoias are incredibly important both for what they are and as symbols of the National Park System," park spokesman Scott Gediman said Saturday.
On Saturday night, officials in San Francisco were monitoring the situation closely.
The fire is threatening some of San Francisco's vital utilities. The city owns power lines just outside the fire zone, and 85 percent of its water comes from the Hetch Hetchy reservoir, which is about 4 miles from the inferno.
The flames have already shut down two of the city's hydroelectric power plants. Gov. Jerry Brown has declared a state of emergency.
The Rim Fire has burned 203 square miles — an area about the size of Chicago — and was just 7 percent contained late Saturday night. It started in a remote canyon of the Stanislaus National Forest Aug. 17 and grew rapidly under dry conditions.
The fire has grown so large it is difficult to predict in which direction it will move. Steep, inaccessible terrain was hampering firefighter's effort to surround it.
With 30 to 40 mph winds expected to push the fire further north into the park Sunday, fire crews were focused on attacking its northern edge to keep flames from the communities of Tuolumne City, Twain Harte and Long Barne.
"The wind could push it further up north and northeast into Yosemite and closer to those communities and that is a big concern for us," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.
The U.S. Forest Service says about 4,500 structures are threatened. Berlant said 23 structures were destroyed, though officials have not determined whether they were homes or rural outbuildings.
Jessica Sanderson said one of her relatives gained access to the family's property in Groveland, just 26 miles from the park's entrance, on Saturday and was able to confirm their vacation cabin had burned to the ground.
The family saw firefighters defending the cabin on a TV news report just a day earlier.
"It's just mind-blowing the way the fire swept through and destroyed it so quickly," said Sanderson, who's been monitoring the fire from her home near Tampa Bay, Fla. "The only thing left standing is our barbeque pit."
The tourist mecca of Yosemite Valley, the part of the park known around the world for such sights as the Half Dome and El Capitan rock formations and waterfalls, remained open, clear of smoke and free from other signs of the fire that remained about 20 miles away.
More than 2,600 firefighters and a half dozen aircraft were battling the blaze.
The fire was also burning toward the Hetch Hetchy reservoir, where San Francisco gets 85 percent of its water, and power for municipal buildings, the international airport and San Francisco General Hospital. Gov. Jerry Brown declared a state of emergency because of the threats.
Officials with the San Francisco Public Utilities Commission were running continuous tests on water quality in the reservoir that is the source of the city's famously pure water.
Deputy General Manager Michael Carlin said that no problems from falling ash have been detected.
"We've had other fires in the watershed and have procedures in place," he said.
The commission also shut two hydro-electric stations fed by water from the reservoir and cut power to more than 12 miles of lines. The city has been buying power on the open market.