Hundreds of people have been forced to leave their homes in mountains north of Los Angeles, California, the authorities say.
The fast-moving wildfires had covered an area of 20,000 acres by July 23, sending a pall of smoke across parts of Los Angeles County.
About 300 people have been evacuated near the city of Santa Clarita.
Public swimming pools in Pasadena and Glendale closed because of smoke and falling ash.
The wildfires, known as the Sand Fire, are being driven by high temperatures and strong winds, as forecasters warn the conditions are set to continue.
They broke out on July 22 in the Sand Canyon area near Santa Clarita and winds quickly fanned them towards the Angeles National Forest.
About 900 firefighters are battling the flames, helped by helicopters and planes dumping water and fire retardant.
According to LA County Deputy Fire Chief John Tripp, about 1,000 homes were currently in danger but if the situation worsened, up to 45,000 homes, mostly in the San Fernando Valley, could be at risk.
Among those evacuated to safety were about 400 animals from the Wildlife Waystation, a sanctuary for rescued exotic animals within the national forest.