Los Angeles, Aug 31 (DPA) A raging California wildfire doubled in size Monday and two firefighters were killed when their vehicle tumbled down a mountain as they were fighting the region's worst blaze of the year.

Some 12,000 homes are currently threatened by the fire, which could also knock out the transmitters towers for much of the television, radio and cellphones in the Los Angeles area.

Further to the south, the Oak Glen Fire threatened 2,000 homes in San Bernardino County as dry and hot conditions across the region fanned flames that raced through tinder-dry brush.

Some 50 helicopters and more than 6,400 firefighters have been mobilised to battle the blazes, which have now burnt over 34,000 hectares since they ignited last week. At least 18 homes were destroyed by Monday morning, but fire officials expected the toll to worsen.

The flames in the Los Angeles National Forest sent a huge plume of smoke into the air over the city and could be seen for miles, while authorities warned of poor air quality.