Most of the roughly 20,000 evacuees forced out by a wildfire have been cleared to go home, but firefighters still faced huge work on Tuesday in taming a wildfire in mountains north of Los Angeles.

The fire's size increased modestly overnight to 150.22 sq kilometres but containment more than doubled to 25% although authorities remained cautious.

"We're not really out of the woods," said US Forest Service spokesman Justin Correll.

"We're not ready to relax. There's still a lot of firefighting to do."

A week of three-figure temperatures awaited the nearly 3,000 firefighters battling flames in rugged terrain between Los Angeles and suburban Santa Clarita, where many homes are tucked into canyon lands.

Residents of two neighbourhoods still under threat had to remain out of their homes, the US Forest Service said.

Eighteen residences have been destroyed in the blaze that started on Friday afternoon and quickly tore through drought-ravaged brush that hadn't burned in decades.

Laurent Lacore was among those who evacuated on Saturday, the last of his family of four to leave as the fire bore down on his house.

"The flames were right behind our backyard," he said.

Mr Lacore was among many who were told they could return on Sunday only to learn on arriving at the scene that new winds and new flames meant more days in a hard-to-find hotel room.

He returned on Monday night delighted to find the house and everything around it had been saved, and could see a line of red fire retardant nearby where a helicopter had stopped the fire's approach.

"Everything is fine," he said. "Even all of the trees are there."

Firefighters saved about 2,000 homes in the fire's first three days, Los Angeles county deputy fire chief John Tripp said.

Some 300 miles to the north-west a blaze in the scenic Big Sur region of the Central Coast had destroyed 20 homes and threatened 1,650 others as it burned 77.7 sq km by Tuesday morning.

However, firefighters made gains on Monday and had it 10% contained.

The two blazes sent smoke as far away as Nevada, where officials issued air pollution warnings.

At the peak of the Southern California fire about 10% of Santa Clarita's 200,000 residents were ordered out of their homes, before most were allowed to return on Monday night.

The fire exploded over the weekend like a "crazy storm," said Kara Franklin.

Ms Franklin said sand driven by heavy winds hit her in the face as she tried to get a horse and donkey into a trailer so she could tow the animals away. From a ridgetop, she saw flames engulf a neighbourhood.

"The heat was so intense," she said.

A house two doors from hers was engulfed, providing a buffer that helped save her house.

Three Forest Service firefighters lost their homes at a remote fire station in the San Gabriel Mountains, including two who were fighting the fire at the time.

Investigators were trying to determine the cause of death of a man whose body was found in a car in the fire zone on Saturday.