A firework at a gender reveal party triggered a wildfire in southern California that destroyed 7,000 acres and forced many residents to flee their homes.

More than 500 firefighters and four helicopters battled the El Dorado blaze east of San Bernardino, which started on Saturday morning, said California Department of Forestry and Fire Protection (Cal Fire).

Residents of several communities in the area have been ordered to evacuate.

"CAL FIRE Law Enforcement has determined the El Dorado Fire, burning near Oak Glen in San Bernardino County, was caused by a smoke generating pyrotechnic device, used during a gender reveal party," Cal Fire said on Twitter.

"Those responsible for starting fires due to negligence or illegal activity can be held financially and criminally responsible," it warned.

Gender reveal parties are held during pregnancy to unveil the sex of the expected child, sometimes announced by pink or blue smoke fireworks.

With hot and dry conditions, California has suffered a particularly busy fire season this year, with more than 1.6 million acres destroyed so far, including in three of the biggest blazes in the state's history.

In northern California, more than 200 people had to be airlifted to safety over the weekend after a fast-moving wildfire trapped them near the Mammoth Pool Reservoir northeast of Fresno.

The Creek Fire, which started on Friday in steep and rugged terrain, has so far spread to 45,500 acres, according to the US Forest Service, making it one of the largest blazes.

The blaze was 0% contained on yesterday afternoon, while nearly 15,000 firefighters were battling some two dozen fires across the state, according to the Cal Fire.

A rapidly-spreading fire near San Diego, Valley Fire, has so far spread to 10,000 acres and destroyed 11 structures, it added.

Eight people have been killed and around 3,300 structures have been destroyed over the past three weeks in wildfires across the state.

Three major fires, including the Creek Fire, were burning in Fresno, San Bernardino and San Diego counties, Cal Fire said in a statement, adding it had increased staffing in preparation for "critical fire weather".

Record temperatures over the three-day Labor Day weekend have aggravated already dangerous fire conditions and further stressed exhausted California firefighters.

Temperatures reaching more than 43C were forecast for the Los Angeles area today.

The temperature reached 49C yesterday afternoon, a record for the National Weather Service office that covers the metropolitan area.

"This is an historic heat wave for south western CA and one that will be remembered for a long time," the service said in a statement.