The two biggest wildfires ravaging the Los Angeles area have killed at least 10 people and burned more than 10,000 homes and other structures, officials say as they urged more people to heed evacuation orders after a new blaze ignited and quickly grew.

The Kenneth Fire started in late Thursday afternoon in the San Fernando Valley just 3.2 km (2 miles) from a school serving as a shelter for evacuees from another fire. It moved into neighbouring Ventura County but a large and aggressive response by firefighters stopped the flames from spreading.

About 400 firefighters remained on scene overnight to guard against the fire flaring up.

Only hours before the Kenneth Fire roared to life officials expressed encouragement after firefighters aided by calmer winds and help from crews from outside the state saw the first signs of successfully beating back the region’s two devastating wildfires.

The Palisades Fire between Santa Monica and Malibu on the city's western flank and the Eaton Fire in the east near Pasadena already rank as the most destructive in Los Angeles history, consuming 34,000 acres (13,750 hectares) – or some 53 square miles – turning entire neighbourhoods to ash.

The death toll from the blazes rose to 10 from seven, Los Angeles County's Medical Examiner said in an update.

Los Angeles County Sheriff Robert Luna told an earlier press conference he expected the number to grow.

"It looks like an atomic bomb dropped in these areas. I don't expect good news, and we're not looking forward to those numbers," Luna said.

Officials said the Eaton Fire had damaged or destroyed 4,000 to 5,000 structures while the Palisades Fire destroyed or damaged another 5,300 structures.

Private forecaster AccuWeather estimated the damage and economic loss at $135 billion to $150 billion, portending an arduous recovery and soaring homeowners' insurance costs.

"We're already looking ahead to aggressively rebuild the city of Los Angeles," said Mayor Karen Bass, a Democrat, who faced criticism from President-elect Donald Trump and other Republicans over her handling of the disaster.

Southern California Edison said it had received notices from insurance companies to preserve evidence related to the Eaton Fire, but said no fire agencies have pointed to the utility's connection to the fire.

"We're already looking ahead to aggressively rebuild the city of Los Angeles," said Mayor Karen Bass, a Democrat, who faced criticism from President-elect Donald Trump and other Republicans over her handling of the disaster.

Scenes of massive destruction and devastation in Pacific Palisades. – Reuters

President Joe Biden, who declared a major disaster on Tuesday, promised that the federal government would reimburse 100 per cent of the recovery for the next 180 days to pay for debris and hazard material removal, temporary shelters and first responder salaries.

"I told the governor, local officials, spare no expense to do what they need to do and contain these fires," Biden said.

In all, five wildfires burned in Los Angeles County, including one rapidly growing blaze that crossed the border from Ventura County. The skies buzzed with aircraft dropping retardant and water on the flaming hills.

A large Super Scooper aircraft on loan from Canada was damaged and grounded after hitting an unauthorized civilian drone near the Palisades fire, the L.A. County Fire Department said. There were no injuries.

One rapidly growing blaze broke out near Calabasas, one of the wealthiest cities in the US and home to numerous gated communities. 

The so-called Kenneth Fire expanded to 960 acres (388 hectares) in a matter of hours.

With nerves on edge, Los Angeles County mistakenly sent an evacuation notice countywide to a population of 9.6 million, County Supervisor Janice Hahn said, but a correction was quickly sent.

"We are alive. That's all that matters": Security guard describes Palisades devastation. – Reuters

Some Pacific Palisades residents ventured back to areas the fire had already swept through, where brick chimneys were left looming over charred waste and burnt-out vehicles.

"We are alive. That's all that matters," private security guard Bilal Tukhi said while standing watch outside his employer's damaged home, saying the scene reminded him of his native, war-torn Afghanistan.

School was cancelled for a second day on Friday due to the smoke, ash and particulates that contaminated the air, Superintendent Alberto Carvalho said.

The Eaton Fire's growth has been significantly stopped, Marrone said, though it remains 0 per cent contained. While still strong, winds have diminished since the 100mph (160km/h) gusts seen earlier in the week, permitting crucial aerial support for crews on the ground.

But officials warned that the wind was forecast to intensify again overnight, and red flag conditions were expected until Friday afternoon.

Drone video shows scope of devastation from fire in Altadena, and residents return to homes destroyed by wildfires. – AP

The Eaton Fire reached the grounds of the Mount Wilson Observatory, the place where a century ago Edwin Hubble discovered the existence of galaxies beyond the Milky Way.

"There are firefighters on the grounds but the power is out at the Observatory so communication is limited. We are monitoring the situation and will update here as we can," the observatory said.

The two biggest conflagrations – the Palisades and Eaton fires – formed a pincer around the city so enormous that it was visible from space.

In Pacific Palisades, an upscale and picturesque enclave, once-palatial homes stood in ruins, while downed power lines and abandoned cars littered the roadways.

The smell of heavy smoke filled the air, and residents wearing masks rode bicycles, hoping to catch a glimpse of their damaged houses.

Aerial video showed block after block of levelled homes, while satellite images showed the two largest fires forming a pincer around the city and thick plumes of smoke from the fires being blown out over the Pacific Ocean.

A walk through wildfire devastation in Pacific Palisades. – Reuters

Firefighting crews managed to control the Sunset Fire that forced mandatory evacuations in Hollywood and Hollywood Hills.

Flames raged atop the ridge overlooking Hollywood Boulevard's Walk of Fame on Wednesday night as fleeing residents created traffic jam.

Firefighters from half a dozen other US states were being rushed to California, while an additional 250 engine companies with 1000 personnel were being moved from Northern California to Southern California.

Canadian Prime Minister Justin Trudeau, whose country has experienced its own severe wildfires, sent water bombers to Southern California to help, and said 250 Canadian firefighters were ready to deploy.

The Canadian Armed Forces were standing by to move personnel and equipment."To our American neighbours: Canada's here to help," Trudeau said.

Homes, church destroyed in fire-ravaged Altadena. – AP