More than 2,200 firefighters were battling a blaze that was burning out of control Monday in southern California, threatening thousands of people and homes east of Los Angeles.

The Apple Fire, which broke out Friday near the city of San Bernardino, has so far burnt more than 20,000 acres (8,000 hectares), sending up columns of smoke visible for miles.

The so-called Apple Fire has charred more than 32,000 acres of land.
The so-called Apple Fire has charred more than 32,000 acres of land. AFP / JOSH EDELSON

Record low moisture in the vegetation, low humidity and high temperatures are fuelling the blaze, the United States Forest Service said.

At least 2,600 homes and nearly 7,800 people were evacuated. Officials said it was not clear when they might be able to return.

By early Monday about 2,260 firefighters backed by helicopters, water-dumping planes and trucks were working to put out the fire.

A probe is under way as to the cause of the fire, which officials said may have been set deliberately
A probe is under way as to the cause of the fire, which officials said may have been set deliberately AFP / JOSH EDELSON

Crews managed to contain five percent of the fire by Monday, after losing control of the blaze during Sunday afternoon.

There were no reports of casualties, and the only damage so far has been to two buildings and one home.

The edges of the fire are burning on rugged hills hard for crews to reach, authorities said.

Dense vegetation fueled the blaze near homes, said Fernando Herrera, fire chief in Riverside County, while hot and dry conditions Monday will help the flames keep alive.

Officials said the fire may have been set deliberately and a probe is underway.