LOS ANGELES -- A cooking fire at a homeless encampment sparked a wildfire last week that destroyed six homes in the Bel Air neighbourhood of Los Angeles, authorities said Tuesday, while the fifth-largest wildfire in California history burning northwest of the city kept expanding and kept thousands out of their homes.

They are among a half-dozen fires that flared in Southern California last week and were driven by fiercely gusting Santa Ana winds.

Arson investigators determined that the Bel Air fire near the world-famous Getty museum was started by an illegal fire at a camp near a freeway underpass, city fire Capt. Erik Scott said.

The camp was empty when firefighters found it but people apparently had been sleeping and cooking there for at least several days, he said.

Northwest of Los Angeles, firefighters protected foothill homes while the fire grew mostly into forest land, Santa Barbara County Fire Department spokesman Mike Eliason said.

Red Flag warnings for fire danger due to Santa Ana winds and a critical lack of moisture were extended into the week, with a possible increase in gusts Thursday into Friday.

Tens of thousands of people remain evacuated, including many from the seaside enclaves of Montecito, Summerland and Carpinteria and the inland agricultural town of Fillmore.

Still among evacuees due to smoke Tuesday were Rock and Roll Hall of Fame member Eric Burdon and his wife, Marianna, of Ojai. Last week, Burdon wrote on Facebook about having to flee and returning temporarily to find their home still standing with ashes all around.

"A week like this gives you the perspective that life is what truly matters," he wrote.

A photo accompanying the post showed his handprint and signature written in ashes.

Residents near a Carpinteria avocado orchard said the trees could end up saving their homes.

"You have a thick layer of leaves underneath the bottom and they are watered regularly, so it's like a sponge," Jeff Dreyer, who lives nearby, told KEYT-TV. "So the fire gets to the sponge full of water and it slows it down."

Officials handed out masks to those who stayed behind in Montecito, an exclusive community about 75 miles (120 kilometres) from Los Angeles that's home to stars such as Oprah Winfrey, Jeff Bridges and Drew Barrymore. Actor Rob Lowe was among residents who evacuated over the weekend.

The blaze -- known as the Thomas Fire -- has destroyed more than 680 homes, officials said. It was just partially contained after burning more than 360 square miles (930 square kilometres) of dry brush and timber. The fire has been burning for more than a week.

To the north, San Francisco Bay Area firefighters quickly contained blazes Tuesday that destroyed at least two homes in hills east of Oakland -- the site of a 1991 firestorm that killed 25 people.