One killed and 50 homes destroyed in US gas explosion

"If it is ultimately determined that we were responsible for the cause of the incident, we will take accountability," Pacific Gas and Electric Co said.

Victims with serious burns began arriving at San Francisco Bay area hospitals shortly after the blast. An estimate of the number of injured was not immediately available.

Hospitals reported receiving about 20 injured patients - several of whom were in a critical condition - and they anticipated getting more.

Flames hot enough to crack a fire engine windshield incinerated 53 homes and damaged 120 as crews continued battling the blaze into the night. At least one person was confirmed dead in the blast, San Bruno Fire Chief Dennis Haag said.

The fire had spread to 10 acres, said Jay Allen, spokesman for the California Emergency Management Agency.

Between 150 and 200 firefighters were at the scene, Mr Haag said. More than 100 people were being sheltered at nearby evacuation centres, but no estimate of the number of residents missing was available.

Witnesses said the explosion shot a fireball more than 1,000 feet in the air and sent frightened residents fleeing for safety and rushing to get belongings out of burning homes.

Planes and helicopters flew over the neighbourhood dumping water.

San Bruno Fire Captain Charlie Barringer said the neighbourhood was engulfed by the time firefighters arrived, even though the fire station was only a few blocks away. He said the blast took out the entire water system, forcing firefighters to pump water from more than two miles away.

Mr Haag said firefighters initially had trouble getting close enough to the ruptured gas line to shut it down because of the flames.