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

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.

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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.

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A firefighter battles California's Rim Fire near Yosemite National Park on 26 August 2013