Video: Suspected gas leak behind San Francisco explosion
A massive blast triggered by a suspected gas leak has killed at least four people and destroyed more than 50 houses in a neighbourhood near San Francisco.
The explosion shot a fireball more than 1,000 feet in the air, sending residents fleeing for safety and rushing to get belongings out of burning homes, witnesses said.
Fire crews flooded the ruins of burning houses with water as flames reached as high as 50 metres.
Utility officials said a natural gas line had ruptured nearby.
The blast left a giant crater and sent flames tearing across several blocks in San Bruno just after 6pm.
Coroner Michelle Rippy said officials at the scene confirmed at least four deaths. At least 20 others were injured, some with critical burns.
California Lieutenant Governor Abel Maldonado, acting governor while Arnold Schwarzenegger was in Asia on a trade mission, declared a state of emergency in San Mateo County.
"It was like a bomb went off," Mr Maldonado told CNN.
He said the cause of the blast was still unclear.
Pacific Gas and Electric Co said a damaged section of a 30-inch steel gas pipeline had been isolated and the gas flow had been stopped.