A state of emergency is in force in San Francisco over fears that a raging wildfire could cripple the city's water and power services.

The authorities in California say the giant wildfire poses a serious threat if it continues to spread.

The fire has been burning for over a week.

It is still no closer to being controlled, despite the efforts of over 2,500 fire-fighters.

The outbreak which has affected parts of Yosemite National Park is reported to be closing in on a reservoir providing drinking water to millions of people.

Mountainous terrain is hampering efforts to control the blaze putting thousands of homes at risk.