San Francisco  - California authorities stepped up their battle Friday against the more than 1,000 fires that have cast a blanket of smoke over much of the state, sending more than 12,500 firefighters to fight the flames.

According to state firefighting organization Cal Fire, 1,211 fires had burned more than 780 square kilometers, and only 266 fires have been contained.

Most of the fires were sparked when freak dry lightning storms hit much of northern California last weekend, and meteorologists have predicted more of the same weather conditions this weekend. However the army of firefighters, backed by about 1,000 fire engines and some 100 water-dropping aircraft appeared to be making progress.

"There's been a lot of good work out there, and we're definitely making progress on a lot of these fires," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.

One of the worst blazes was burning out of control near the renowned coastal beauty spot of Big Sur, having already burned 15 houses and threatening 500 more. It also threatened a historic library at the site, where famous author Henry Miller spent his final years.

Smoke filled skies forced authorities to extend an "unhealthy air" day across the San Francisco Bay region advising seniors, children and people with breathing problems to stay indoors with windows closed to minimize breathing problems.

In the Mendocino region alone, more than 130 fires were burning, forcing hundreds from their homes, while in the Shasta Trinity forest more than 150 fires were burning.

Initial reports were of little damage to property but more than 2,000 homes are under threat from the conflagration.

The fires are causing huge concern coming early in the season due to low rainfall over the winter and spring that left brush bone dry at least two months ahead of schedule.

California's worst fire months are usually in September and October, when hot dry inland winds blow across the south of the state. Last year more than 2,000 homes were destroyed by the late summer fires, causing more than 3 billion dollars in damage.

California Governor Arnold Schwarzenegger declared a state of emergency earlier in the month and called in the National Guard Wednesday to boost firefighting efforts.

He called on people not to buy fireworks for the traditional July 4 US Independence Day celebrations, as it was too dry and too dangerous to set them off. (dpa)