By Alex Dobuzinskis

LOS ANGELES (Reuters) - A 4,000-acre (1,620-hectare) wildfire in the Angeles National Forest in Southern California has forced the evacuation of some campsites as firefighters struggle to control the blaze, authorities said on Monday.

The fire amid dry chaparral was only 5 percent contained and heavy smoke was visible from miles away. Authorities said it was not threatening any structures.

The fire broke out on Sunday in the San Gabriel Canyon area of the 655,000-acre (265,000-hectare) Angeles National Forest. About 12,000 weekend visitors, hikers and others were evacuated from the area, said U.S. Forest Service spokeswoman Angie Lavell.

The Angeles is less than 15 miles from downtown Los Angeles at its southern end. It is one of the most heavily visited national forests, with about 30 million annual visitors.

Sites outside the fire zone were open to visitors on Monday.

Winds were driving flames toward the Sheep Mountain Wilderness, an area not visited by people, Lavell said.

Six air tankers and eight helicopters were helping about 500 firefighters battle the blaze. The fire is thought to have started in the Camp Williams trailer park and the cause is under investigation, Forest Service spokeswoman L'Tanga Watson said.

The sites evacuated in the San Gabriel Canyon area on Sunday included popular recreation areas along the San Gabriel River and a place for off-road vehicles.

In 2009, a fire in the Angeles National Forest burned more than 160,000 acres. A large swath of the charred forest was closed to visitors for about two years.

(Reporting By Alex Dobuzinskis; Editing by Peter Cooney)