Multimillion-dollar properties, some owned by Hollywood celebrities, were in the path of the blaze, which exploded late Monday, fanned by powerful winds and tinder-dry brush. Firefighters were unable to get a foothold in the steep canyons that surround the tony enclave near Los Angeles, where towering flames were devouring hillsides, blanketing the air in choking smoke.

Law enforcement officers have ordered thousands of people to leave their homes, with many more told to be ready to flee if conditions worsen.

“There are approximately 20,000 affected citizens under evacuation orders and evacuation warnings,” said Captain Jennifer Seeto of Los Angeles County Sheriff. “Fire can be extremely unpredictable, and we highly encourage residents in the fire areas to be prepared. Have a plan and heed our evacuation warnings and orders.”

By Tuesday evening, the so-called Franklin Fire had swelled to more than 2,800 acres (1,150 hectares) and was still raging out of control, fanned by powerful seasonal winds that have led to forecasters hoisting a “red flag warning.” Los Angeles County fire chief Anthony Marrone said 1,500 firefighters were continuing to battle the flames, with the help of aircraft that were dumping thousands of gallons (litres) of water and retardant from the air. Aerial reconnaissance showed an unconfirmed number of homes damaged and destroyed, he told reporters.

Tens of thousands of people across southern California had their electricity cut off. Utilities frequently de-energize lines ahead of windy days to reduce the fire risk from downed power lines.