The wildfire threatening Yosemite National Park is spreading further into the US tourist landmark, officials said as they battled to stop it clouding a holiday weekend.
Efforts to contain the so-called Rim Fire, which has grown to become California's sixth biggest wildfire ever, were also being boosted by the deployment of a military drone approved by the Pentagon.
The fire, which now covers more than 192,000 acres, or 300 square miles, and is 30 percent contained, has also threatened San Francisco's water supply, due to ash falling on a key reservoir.
The blaze, about a quarter of which is now inside the park's boundaries, "is expected to continue its eastward spread farther into the west side of Yosemite National Park," said the latest firefighters' online update.
The fire, which started west of the park on August 17, is threatening some 4,500 structures and on Wednesday forced the closure of a second main road into the major US tourist attraction ahead of the Labor Day weekend.
A surge of visitors is typically expected over this weekend's Labor Day holiday at Yosemite, which draws millions of tourists every year, most in July and August. Labor Day traditionally marks the end of the summer season.
"On a normal holiday weekend we see anywhere between 15,000-21,000 people ... I still expect that it will increase but just not quite as much as it usually does," Yosemite spokeswoman Kari Cobb told AFP.
Last week officials closed Highway 120, the main road used by visitors from San Fransisco into the west of the park. There are three other main roads in.
On Wednesday authorities closed a key section of the Tioga Road, which runs horizontally across the park, effectively blocking access from the east into Yosemite Valley, the spectacular area at its heart.
The company which runs hotels, dining and other facilities in the park said it had had some cancelations due to the fire.
"We have been fortunate that the smoke has not moved to this direction since the fire began," Lisa Cesaro, spokeswoman for DNC Parks & Resorts at Yosemite, told AFP.
"We are still anticipating a good holiday weekend in Yosemite. Most of our lodges are near sell out on Saturday, but we have some availability for last-minute reservations for the next few weeks.
"We have received some cancellations or reservation changes due to the Rim Fire and road closures, but we also have received new reservations as people learn there is availability."
By Thursday the fire had burned through some 45,000 acres inside the park, or 23 percent of the fire's total area, she said. But there was no risk that it will reach Yosemite Valley, the park's main tourist area which is still 15 miles away.
"It would have to travel a very far distance to get here. And the other thing is that Yosemite Valley is surrounded by granite cliffs, and granite is a very good fire stopper.
"So at this point there is absolutely no talk of the fire reaching Yosemite Valley."
The inferno has however reached the Hetch Hetchy Reservoir, the main source of water for 2.6 million people living in the San Francisco Bay Area, some 200 miles (320 kilometers) to the west.
Ash has fallen on the reservoir, but water quality has not been affected so far, authorities say.
No injuries or deaths have been reported due to the blaze, but it has destroyed at least 111 structures—31 homes as well as buildings on campgrounds that were hastily evacuated last week when the fire erupted.
The California National Guard on Wednesday began using a Predator military drone, of the kind which have been used in Afghanistan over the last decade, to monitor the blaze from the air, a Pentagon spokesman told AFP.
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