Firefighters in California are struggling to gain control of a huge wildfire which has reached the edge of Yosemite National Park.
The Rim Fire covers nearly 234 sq miles (606 sq km) and threatens a major reservoir serving San Francisco.
The fire is 15% contained after burning for more than a week, officials say, up from 2% containment on Friday.
Strong winds have fanned the flames closer to nearby towns raising the risk of flying debris sparking more fires.
"This fire has continued to pose every challenge that there can be on a fire," said Daniel Berlant of the California Department of Forestry and Fire Protection.
"It's a very difficult firefight."
More than 5,000 homes are endangered by the blaze, which began on 17 August from still unknown causes in the Stanislaus National Forest.
San Francisco's water threatenedSome 2,800 firefighters are tackling the flames in difficult terrain.
Evacuations, some voluntary and some mandatory, are taking place. Despite the threat to thousands of homes, only a few have been destroyed.
California Governor Jerry Brown has declared a state of emergency for San Francisco 150 miles (220km) away as the blaze is also threatening power lines that bring electricity to the city.
The city's water supplies could be affected if the flames reach Hetch Hetchy reservoir, which supplies San Francisco with 85% of its water. Ash is reportedly falling on the reservoir like snow and visibility is limited to 100ft (30m), but officials say the water quality is still good.
The fire is burning some four miles away from the reservoir which serves some 2.6 million customers.
Two of three hydroelectric power stations serving the city were shut down, forcing the San Francisco Public Utilities Commission to spend $600,000 (£385,000) buying power on the open market.
The blaze, which continued for a week on the edges of Yosemite, reached the park's backcountry at Lake Eleanor on Friday. But it remains some 20 miles away from Yosemite's main tourist area.
The park authorities say they have no plans to close the attraction as most of Yosemite, which hosts up to 15,000 visitors a day in the summer, is unaffected by the fire.
Areas on the north-western edge of the park, including that round the Hetch Hetchy reservoir and Lake Eleanor, have been closed throughout the week.
A stretch of motorway, Highway 120, which is one of three entrances to the west side of the park, remains closed. Visitors are being urged to use alternative routes from the west.
The Rim Fire is one of 50 major wildfires burning in the western US. Lack of rain and snow have made it a bad year, with 5,700 fires being tackled so far.
The Beaver Creek fire in Idaho has destroyed some 45,000 hectares near the ski resort of Sun Valley.
Meanwhile, five wildfires in Yellowstone National Park have scorched about 18 sq miles of mostly remote areas.
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