LOS ANGELES (AFP, REUTERS) - Thousands of firefighters were battling one of the most destructive wildfires in California's history on Monday (Dec 11) as it crept relentlessly up the Pacific coast and forced new evacuations.

Together, the six major wildfires in California state are larger than the areas of New York City and Boston combined, reported CNN.

US President Donald Trump has issued a state of emergency for California, authorising the release of federal funds. The White House said Agriculture Secretary Sonny Perdue briefed the president and Vice-President Mike Pence on the response to the wildfires and prevention efforts.

As a wildfire near Los Angeles was brought under control, fire crews were being redeployed to battle the Thomas Fire north-west of America's second-largest city.

Fixed-wing aircraft and helicopters were assisting nearly 6,400 firefighters trying to keep the Thomas Fire away from beachfront towns south of the historic city of Santa Barbara, the California Fire Department (Cal Fire) said.

"Gusty north-east winds will cause the fire to threaten areas of the city of Santa Barbara," which has a population of 91,000, Cal Fire said.

Eric Boldt, a meteorologist with the National Weather Service in Los Angeles, said he did not see any immediate respite to the dry, windy conditions that are fuelling the blaze.

"The conditions are very critical," Boldt said.

"Given that we don't see any precipitation for the next two weeks at minimum, it could become the largest fire in the state's history."

The Thomas Fire has burned 93,300ha since it erupted a week ago, Cal Fire said, making it the fifth-largest fire in the state's history.

It was only 15 per cent contained as of early afternoon on Monday.

The blaze spread over the weekend in the mostly uninhabited Los Padres National Forest, and was threatening the coastal towns of Montecito, Summerland and Carpinteria.

Mandatory evacuation orders were lifted for the inland cities of Ojai and Santa Paula in Ventura County, which has been the hardest hit by the Thomas Fire.

Comedian and talk show host Ellen DeGeneres was among those preparing to flee her beachfront home.

Oprah Winfrey is reported to have homes in the wealthy enclave of Montecito just outside Santa Barbara.

'New normal'

May Osher, a 66-year-old retired schoolteacher in Carpinteria, told the Los Angeles Times she had put photo albums and pet supplies in her car and was ready to evacuate if told to do so.

"I'm staying until it's time to go," Osher told the newspaper.

Cal Fire said 800 buildings have been destroyed by the fire, which is being fuelled by tinder dry brush and strong and unpredictable Santa Ana winds.

Despite the intensity of the fires stretching from north of Los Angeles down to San Diego, the authorities have reported only one fatality - a 70-year-old woman who died in a car crash while fleeing a blaze.

California Governor Jerry Brown has warned that climate change meant the state was becoming increasingly vulnerable and wildfires were becoming the "new normal".

"The fire season used to be a few months in the summer," Brown said. "Now it's almost year-long."

Brown has been one of the most vocal critics of President Trump's decision to pull the United States out of the Paris accord on global warming, and he renewed his attacks on the administration over its attitude towards climate change.

"Nature is not a political game. Nature is the ground on which we stand, it's the air which we breathe," he said.

"The truth of the case is that there's too much carbon being emitted, that heat-trapping gasses are building up, the planet is warming and all hell is breaking loose."

Santa Barbara Mayor Helene Schneider, speaking by phone, said winds have calmed a bit, keeping the fire in the hills outside of the city. The fire was not spreading quickly, but it also is not clearing out smoke.

"Everywhere you go, you see people wearing the masks," said Schneider.

A layer of ash is also gathering over the city, she said.

This has been California's deadliest year ever for wildfires. More than 40 people died in October when fires swept through the state's wine-producing counties north of San Francisco.