At least four people have died and 52 have been injured after a gas pipeline exploded and set off a fire that engulfed a neighbourhood near San Francisco.

Earlier reports put the number of dead at six, and the death toll is expected to rise as more homes are searched, the Los Angeles Times reported.

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More than 100 residents were forced to flee and more than 170 homes damaged or destroyed as the blast ripped through the town of San Bruno and triggered a giant fire.

Up to 200 firefighters are tackling the blaze which was still raging before dawn, more than nine hours after the initial explosion.

The heat of the blaze was so intense that it melted fire truck windows and car taillights several dozen metres from the flames.

California's acting governor Abel Maldonado declared a state of emergency for the city just 3km west of San Francisco International Airport.

Pacific Gas & Electric spokesman Jeff Smith described the fire as 'an absolutely horrible situation', and said that if it turns out the utility was to blame, 'we will absolutely take accountability for it'.