Twleve people have been confirmed dead on Sunday in two fires in San Diego, south of Los Angeles. Two other wildfires threatened to merge near San Bernadino, about 60km east of Los Angeles, meanacing nearly 24,000 homes, authorities said.
San Diego County Sheriff, William Kolender, said parts of the city of 1.2 million inhabitants were being evacuated under threat from two separate fires, which have burned 37,600 hectares (93,000 acres) and destroyed 136 homes.
He told residents to "not hesitate to pack your belongings and leave" if the fire came their way and to listen closely to broadcasts from local authorities.
"This is a very serious situation. Pay attention to what you hear from law enforcement, pay attention to your television set," he said.
The fires also forced the evacuation of a federal air traffic control facility, causing flight delays and cancellations, San Diego Airport spokesman Steve Shultz said.
Smouldering remains of a luxury
hillside home in Claremont
"Air traffic control (for San Diego) was transferred to the Los Angeles radar control centre and because of that, there have been delays and cancellations," Shultz said.
Meanwhile, firefighters were failing in their efforts to get the upper hand over two other blazes, dubbed the Old Fire and the Grand Prix, in the Los Angeles suburb of San Bernardino, a city of 185,000 people.
The two wildfires were threatening to merge to form the biggest conflagration in the region in a generation, Norm Walker, incident commander in the San Bernardino National Forest told NBC television.
"It's raging everywhere you can imagine," said Carol Beckley, a fire information officer for the US Forest Service. She said the two blazes combined were the biggest since the early 1980s.
Out of control
The US Forest Service said the evacuation of mountain communities was a priority, as emergency services fought to stop the fires spreading into San Bernadino.
The service warned the fires were out of control and likely to spread in the high winds and dry conditions.
At 1900 GMT, the Old Fire had already reached 6000 hectares, while the Grand Prix had also grown to 10,000 hectares, the Forest Service said. The fires also had forced the closure of two major freeways.
Residents were dousing their homes with water from garden hoses in the hope of saving their property, but Walker said leaving the area was the more prudent course of action.
"We try to encourage everyone to evacuate. People could be killed defending their homes," he said. "There's nothing worth dying for in their homes."