AN intentionally set wildfire has grown perilously close to homes in southern California as evacuation orders expanded to more than 20,000 residents.

Firefighters fought a desperate battle to stop the fire from reaching homes as the blaze surged through the Cleveland National Forest above the city of Lake Elsinore and its surrounding communities.

They were trying to keep the flames from devouring neighbourhoods and taking lives, as gigantic fires still burning in Northern California have done.

Governor Jerry Brown declared a state of emergency for Orange and Riverside counties because the fire threatened thousands of homes.

Fire crews worked in 38C heat. Aircraft dumped lines of fire retardant to wall off the fire from homes. The 16-square-mile blaze was only 5% contained.

Since starting on July 27, the fires – which are being fought together as a single incident – have destroyed more than 100 homes near Clear Lake and become the largest fire in state history.

The area burned by the blaze is now larger than the cities of Los Angeles.