The mayor of Los Angeles, Antonio Villaraigosa, has meanwhile warned that the fire has brought down power lines and could cause rolling blackouts.
"We're at the mercy of the wind. Mother Nature's not been too good to us for the last 15 hours," he said.
Mr Villaraigosa urged people living near the fire zone not to wait for the flames to approach before leaving.
"If you wait for the fire to get there, you waited too long," he said.
No deaths have so far been reported, although Los Angeles Police Chief William Bratton said he feared bodies might be found at the Oakridge Mobile Home Park, where some 500 mobile homes were burnt to the ground overnight.
Mr Bratton said rescue crews would have to wait for the ground to cool before bringing in search dogs to find any victims.
"It was an absolute firestorm," said Los Angeles Fire Department Captain Steve Ruda of the Oakridge blaze.
"Firefighters were braving 50ft flame lengths as they swept across the mobile homes," he told the Reuters news agency, adding that heat from the flames had melted his firefighters' hoses to the road.
The Sylmar fire broke out late on Friday in the foothills of the Angeles National Forest and spread rapidly, whipped up by hot winds of up to 76mph (122 km/h).
California fires 'emergency'
Officials soon ordered about 10,000 residents of Sylmar and nearby Porter Ranch to evacuate their homes as the flames jumped the wide Interstate 5 and Highway 210. Both roads have been closed to traffic.
As of Saturday evening, about 20% of the fire had been contained, fire officials said. In addition to helping spread the fires rapidly, the continued high winds have restricted the use of water-dropping planes.
Critical care patients were evacuated from the Olive View-UCLA Medical Centre as the fire approached at midnight on Friday, causing a three-hour power failure.
More than 1,100 firefighters are battling the blaze as shifting winds pushed it in two directions - toward the San Gabriel Mountains in the national forest and towards the northern suburbs of Los Angeles.
"Near-hurricane winds made it very difficult for firefighters," said Los Angeles Deputy Fire Chief Mario Rueda. "When they arrived, it was very well developed into the forest."
Mr Rueda said firefighters were patrolling the evacuated neighbourhoods, making sure flying embers did not start small fires which later spread.
The wildfire in Montecito, west of Los Angeles, is now about 60% contained
The cause of the Sylmar fire is not yet known. It broke out as crews were still battling another major blaze about 80 miles (130km) to the west, in the wealthy enclave of Montecito, near Santa Barbara.
Thousands of residents have had to flee that fire since it began on Thursday.
More than 180 homes have been destroyed or damaged in the secluded area that is home to celebrities including Oprah Winfrey, Rob Lowe, Michael Douglas and John Cleese.
Multi-million dollar homes have been turned into smouldering piles of rubble surrounded by a grey, ash-coated landscape.
At least 13 people have been treated for burns or smoke inhalation.
In that fire too, hot winds have whipped up the flames as they burnt through brush and trees which have seen little rain in recent months.
It is now about 60% contained, fire officials said.
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