San Francisco fire services battled a massive blaze in a residential part of the city close to the San Francisco International Airport.
A ruptured gas pipeline is believed to have caused the flare-up in San Bruno, which has already consumed between 35 to 45 houses, with flames reaching as high as 18 metres into the air.
No casualties have been reported but hospitals in the area are treating people with burns though it is yet unclear exactly how many people may be injured.
Witnesses say they heard a loud explosion just before the fire started.
Many assumed it was caused by a plane that may have crashed.
The massive fire ripped through a residential area near San Francisco International Airport.
There is speculation that a natural gas pipeline may have ruptured due to underground drilling or construction work.
Footage showed many houses on fire in San Bruno, south of the San Francisco city centre.
Between 35 and 45 homes have either been destroyed or partially burnt, according to early estimates.
Local utility officials are investigating whether the source of the blast was a gas pipeline, although there is no information to suggest works were being carried out in the area.
The explosion generated intense heat and sent a massive fireball into the air, witnesses said.
"I heard a sound like a low-flying plane, then all of a sudden the house shook," said Tina Diloia, who was with her baby in their condominium when the blast occurred about 800 metres away.
"Then there was another explosion. I went outside and there was debris falling from the air.
"There were initial fears that a plane may have crashed, with the nearby airport inundated with calls from concerned residents.
Officials have since ruled out a plane crash or an explosion at a nearby petrol station.
Hundreds of fire fighters are battling to control the flames, with embers spreading to nearby residential properties.
Water drops by helicopters are being carried out in a bid to douse the flames.
CNN reported some people had been injured, while a local hospital spokeswoman told ABC America her hospital was treating one man and had been told another casualty was on the way.
A local pastor told CNN he witnessed a "devastating explosion" that caused the fast-moving fire.
Pastor Leigh Bishop of the Church of Highlands said two people from his church had lost their homes and at least one person had burns injuries.
An emergency shelter has been opened for affected residents.
Emergency officials warned residents to make evacuation plans even if they are not directly affected by the blaze.
