(Corrects paragraphs 1, 2, 10 to show only portion of key road closed, east entrance remains open)
By Laila Kearney
SAN FRANCISCO, Aug 28 (Reuters) - Yosemite National Park, faced with the spread of a massive California wildfire, extended a key road closure in the park on Wednesday in a move expected to hinder visits to one the nation's top outdoor destinations over the Labor Day weekend.
The closure of a long stretch of Tioga Road through the western half of the park comes as the so-called Rim Fire burned deeper into the park and crept closer to the tourist hub of Yosemite Valley.
The blaze, which stands as the sixth largest on record in state history after charring an area larger than Chicago, on Tuesday reached a reservoir that serves as the primary water supply for San Francisco, some 200 miles (320 km) to the west.
Crews were attacking the eastern flank of the fire as it spread toward Yosemite Valley as well as the western edge of the blaze, where some 4,500 homes in a string of small communities stood in the path of the flames, California Department of Forestry and Fire Protection spokesman Dennis Matheson said.
He said that treacherous, often inaccessible terrain was hindering firefighters' efforts to carve additional containment lines around the blaze and estimated it would take another week to fully encircle it.
"I think it's very safe to say that we're looking at least at the first week of September," Matheson said. "A lot of it is footwork, creating containment lines by hand."
Since erupting Aug. 17, the Rim Fire has blackened about 192,500 acres (77,902 hectares) Of dry scrub and timberlands, mostly in the Stanislaus National Forest west of the park. As of Wednesday, 43,000 acres of the total area burned was inside Yosemite, nearly double the figure from Monday.
By late Wednesday, containment lines had been established around 30 percent of the fire's perimeter, up from 23 percent early in the day, according to fire officials.
The flames last week forced the closure of a stretch of Highway 120 that leads into the west side of the 750,000-acre (300,000-hectare) park and is the main entrance from the San Francisco Bay area.
On Wednesday, the closure was extended eastward along 120, also called Tioga Road, as far as Yosemite Creek midway through the park. The rest of the road through the park's east entrance remained open, along with two other gates that allow access from other directions. The expanded closure was imposed to allow fire crews to build containment lines along the road before the blaze approaches, said Yosemite spokesman Tom Medema.
"That will limit the access for visitors to and from the east side of the park, quite possibly over Labor Day weekend, which will have a significant economic impact on the area and (be) an inconvenience for visitors," he said.
Some 4 million people visit Yosemite each year, most of them during the peak months of June through August.
'ERRATIC FIRE BEHAVIOR'
The blaze, the biggest California wildfire since October 2007, is being fought by a force of more than 4,500 personnel, backed by teams of bulldozers and water-dropping helicopters.
Firefighters plan to burn containment lines from the Hetch Hetchy Reservoir in the remote northwestern section of the park south to Tioga Road to stop the fire from moving further east into the park, Medema said.
All remaining campers from the Yosemite Creek Campground and Tamarack Flat Campgrounds were to have been evacuated by Wednesday afternoon, he said. The park also closed the Crane Flat Campground.
The blaze has been among the fastest-moving of dozens of large wildfires raging across the drought-parched U.S. West that have strained national firefighting resources.
After encroaching on the Hetch Hetchy Reservoir for several days, flames reached the edge of the artificial lake on Tuesday. The San Francisco Public Utilities Commission said there was little risk to the reservoir because of the rocky terrain and lack of brush surrounding it.
Officials said some ash had drifted onto the surface of the reservoir, but testing of samples showed water quality remained healthy. If the water should become fouled by too much ash and soot and require filtration, it can be diverted through a treatment plant near San Francisco before being delivered to customers, officials from the commission said.
Most of the homes in the path of the fire have been ordered evacuated or were under advisories urging residents to leave voluntarily or be ready to flee at a moment's notice. The fire has already destroyed dozens of homes and cabins, but no serious injuries have been reported.
The cause of the blaze remained under investigation. (Writing by Alex Dobuzinskis and Dan Whitcomb; Editing by Steve Gorman and Ken Wills)