A new wildfire that broke out north of Los Angeles on Wednesday rapidly spread to more than 38 square km, fuelled by strong winds and dry brush.

Some 31,000 people were under mandatory evacuation orders and another 23,000 face evacuation warnings, Los Angeles County Sheriff Robert Luna told a press conference.

The Hughes fire about 80 km north of Los Angeles further taxed firefighters in the region who have managed to bring two major fires in the metropolitan area largely under control.

Officials warned people in the Castaic Lake area of Los Angeles County that they faced "immediate threat to life," while much of Southern California remained under a red-flag warning for extreme fire risk due to strong, dry winds.