LOS ANGELES -- Fire lines have been established around nearly a third of one of the largest wildfires in California history, but danger remains.

Coastal communities in Ventura and Santa Barbara counties were warned Wednesday that they're still at risk if unpredictable winds whip up again and fan the flames.

The National Weather Service extended warnings through Friday of extreme fire danger conditions throughout much of Southern California due to lack of moisture along with a possible increase in wind gust speeds at the end of the week.

Firefighters made some progress Wednesday on corralling the so-called Thomas Fire, which has spread into national forest land northwest of Los Angeles. However, they warned that the fire would continue to spread west as it eats up parched brush.

By Wednesday evening, state fire officials said the blaze was 30 per cent contained but it continued to threaten Santa Barbara, Carpinteria, Summerland and Montecito -- a wealthy area home to celebrities such as Oprah Winfrey.

Since the blaze broke out on Dec. 4, it has burned more than 372 square miles (965 square kilometres) and destroyed 921 buildings -- including at least 700 homes. It threatens 18,000 buildings and has prompted evacuations of about 100,000 people. It is the fifth-largest wildfire in California history.

To the south in San Diego County, firefighters had come very close to containing another major wildfire a week after it broke out.

That fire burned down 157 structures, most in its destructive first hours.

It also killed 46 race horses at a training centre, and left one of their trainers with serious burns.