Dechert says the fire is less than 5 percent contained and threatening nearby homes, though he couldn't give more details on the threat.
She says 100 Forest Service and county firefighters and five county helicopters are fighting the blaze, with air tankers on the way.
Los Angeles County Fire spokesman Sammy Padilla says the fire started at about 3:15 p.m. when a transformer burst into flames and ignited the brush beneath it.
Santa Clarita is about 40 miles north of downtown Los Angeles.
