Media playback is unsupported on your device Media caption The fires are now threatening San Francisco's water supply, as Alastair Leithead reports.

Firefighters are slowly making progress against a sprawling wildfire that is now 20% contained in and around California's Yosemite National Park.

But the Rim Fire continues to spread and now encompasses more than 288 sq miles (746 sq km), officials say.

The flames are raining ash on a reservoir that supplies water and hydro-electric power to San Francisco.

City officials say they are moving water to lower reservoirs and monitoring supplies for contamination.

The blaze is also threatening thousands of homes and some of California's renowned giant sequoia trees. Some 3,700 firefighters are tackling the flames in difficult terrain.

It has now burned an area larger than the land mass of Chicago, but firefighters hope a cooler forecast toward the end of the week will help contain the blaze.

"We are making progress," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection. He said the expected drop in temperatures "would bring some much-needed relief".

Weather patterns

Media playback is unsupported on your device Media caption Trevor Augustine from Unified Rim Fire Command Group outlines the conditions facing fire crews

The blaze has now destroyed 111 structures - among them 31 homes.

Fanned by strong winds, the fire is burning at the edge of the Hetch Hetchy reservoir, which supplies San Francisco with 85% of its water.

Visibility in the smoke-ridden area is down to 100ft (30m), but ash reportedly falling on the reservoir like snow has not yet reached the water intake valves.

Harlan Kelly Jr, general manager of San Francisco's Public Utilities Commission, said as much water as possible had been brought down from the reservoir and local storage had been replenished.

Officials say that should the water become contaminated, it can be diverted to a water treatment plant to be filtered before being sent into homes.

The blaze has become so large that it was reportedly creating its own weather patterns.

Every year the summer temperatures are a little warmer, hence the conditions for burning are a little more auspicious Hugh Safford, Ecologist, US Forest Service

Karen Nyberg, a US astronaut based at the International Space Station, tweeted a satellite picture of the Rim Fire's giant smoke plumes.

"Every year the summer temperatures are a little warmer, hence the conditions for burning are a little more auspicious," Hugh Safford, an ecologist with the US Forest Service in California, told the Associated Press.

"People can deny it all they want but it's happening. Every year the fuels are a little bit drier."

The blaze began on 17 August in the Stanislaus National Forest from a still-unknown cause. It has now grown to the seventh-largest wildfire in California since 1932.

Evacuations, some voluntary and some mandatory, are taking place. Despite the threat to some 5,000 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.

Image caption Firefighters, like the one seen above from Sacramento, have come from all over the state to battle the blaze. Image caption The Hetch Hetchy reservoir is shrouded in smoke, and flames are reportedly lapping the water's edge. Image caption Most of Yosemite National Park has remained open despite the looming fire, and officials do not plan to close. Image caption Firefighters get ready to monitor the fire's progress through the night. Image caption Tall columns of smoke rising from the Rim Fire can be seen from space. previous slide next slide

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 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.

Two famous stands of the park's giant sequoia trees, the Merced Grove and the Tuolomune Grove, are in the path of the advancing flames and have been closed for several days.

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.