Water and electricity supplies to the USA state of San Francisco are under threat as hundreds of firefighters battle one of California's worst ever wildfires.

California Governor Jerry Brown declared a state of emergency in San Francisco as the blaze - known as the Rim Fire - threatens power lines to the city and a key reservoir which supplies 85% of its water.

The Hetch Hetchy reservoir is under serious threat with the fire just four miles away. It provides water to 2.6 million people.

The 2,800 firefighters battling the blaze have gained little ground in slowing the now 225-square-mile fire. Officials estimate it is just 7% contained.

Hundreds of those firefighters have been deployed to protect mountain communities in the path of the Rim Fire, north of Yosemite National Park, as fierce winds causes flames to jump through tree tops.

Bulldozers are being used to clear lines on the north side of the fire to protect the towns of Twain Hart., Tuolumne City and Ponderosa Hills. 12 helicopters and six planes are dropping water and retardant from the air.

Daniel Berlant, of the California Department of Forestry and Fire Protection, said "A crown fire is much more difficult to fight. Our firefighters are on the ground having to spray up".

"Unfortunately, we are expecting strong winds out of the south. It's going to allow the fire to advance to the northeast. This fire has continued to pose every challenge that there can be on a fire: inaccessible terrain, strong winds, dry conditions. It's a very difficult fire fight" he added.

Investigators are trying to determine how the fire started on August 17th, days before lightning storms swept through the region and sparked other blazes. The Rim Fire has threatened two groves of giant sequoias, which are unique to the region.

The towering trees, which grow only on the western slopes of the Sierra Nevada and are among the largest and oldest living things on earth, can resist fire.

But dry conditions and heavy brush are forcing park officials to take extra precautions in the Tuolumne and Merced groves.