LOS ANGELES (Reuters) - Firefighters were slowly making progress in their battle to contain the inferno that has razed swathes of Los Angeles' Pacific Palisades neighborhood to the ground, but still-spreading flames threatened communities in the populous San Fernando Valley on Sunday.

Aircraft dropped water and fire retardant on steep hills to stem the eastward spread of the Palisades Fire and KTLA television reported that ground crews had managed to save a number of houses, although others were lost.

"LA County had another night of unimaginable terror and heartbreak," Los Angeles County Supervisor Lindsey Horvath said.

Six simultaneous blazes that have ripped across the United States' second-largest city since Tuesday had killed at least 16 people as of late Saturday, the Washington Post and other media reported, citing the Los Angeles County medical examiner's office. Reuters could not reach the coroner.

At least 13 people are estimated to be missing.

The blazes have damaged or destroyed 12,000 structures, fire officials said. The flames have reduced whole neighborhoods to smouldering ruins, destroying the homes of the rich and famous and ordinary folk alike and leaving an apocalyptic landscape.

Over the past 24 hours, the Palisades Fire spread over an additional 1,000 acres (400 hectares), consuming more homes, officials reported.

Cal Fire official Todd Hopkins said that while 11% of the Palisades Fire was now contained, it has burned more than 22,000 acres.

Hopkins told a press conference that fire had spread into the Mandeville Canyon and threatened to jump into Brentwood, an upscale neighborhood that is home to many celebrities, and the San Fernando Valley. It also inched towards the north-south 405 freeway. 

SANTA WINDS

Although the Santa Ana winds that have fanned the flames were calmer on Saturday, the National Weather Service warned that stronger gusts of up to 70 mph (110 kph) could return early next week.

Red Flag warnings remain in effect for L.A. and Ventura counties through to Wednesday, the NWS said.

"These winds, combined with dry air and dry vegetation, will keep the fire weather threat in the area," it said. Conditions were expected to moderate by Thursday.

On Saturday, some residents were allowed to return to the sites of their ruined homes to see what they could salvage from the wreckage. Teams with the L.A. County Sheriff's Department were working to sift through the rubble with sniffer dogs to search for human remains.

Evacuation orders throughout the Los Angeles area now cover 153,000 residents. Another 166,000 residents have been warned that they may have to evacuate, Los Angeles County Sheriff Robert Luna said.

FEDERAL HELP

President Joe Biden spoke by phone with officials to get an update on their efforts and was briefed by aides on federal resources that were being dispatched. 

His declaration of a major disaster unlocked federal assistance for those affected by the wildfires, clearing the way for the Federal Emergency Management Agency to provide support.

Support can range from funding for home repairs to money to replace lost food or medication, FEMA spokesperson Michael Hart said, adding that assistance can be provided within days.

However, President-elect Donald Trump criticized local and state officials he believed had handled the situation badly.   

"The fires are still raging in L.A. The incompetent pols have no idea how to put them out. Thousands of magnificent houses are gone, and many more will soon be lost. There is death all over the place... they just can't put out the fires. What's wrong with them?" he said on his Truth Social media feed.

Los Angeles Board of Supervisors Chair Kathryn Barger told reporters she had invited Trump - who takes office on Jan. 20 - to visit the county to get a first-hand look at the destruction.

WEEKS OF WORK

In Altadena, official Don Fregulia said managing the Eaton Fire and its impact will be a "huge, herculean task" that he said will take "many weeks of work".

The two big fires combined have consumed more than 36,000 acres (14,500 hectares), or 56 square miles (145 square km) - 2-1/2 times the land area of Manhattan - in one of the worst disasters in Los Angeles' history.

Officials have declared a public health emergency due to the thick, toxic smoke and poorer air quality, as the fires lofted traces of metals, plastics and other synthetic materials.

But progress was reported in bringing electrical power back to Los Angeles neighborhoods. Southern California Edison CEO Steven Powell said there were now about 50,000 customers without power, down from more than half a million days earlier.

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

In the Vatican, Pope Francis said in his weekly address to the faithful in St. Peter's Square: "I am close to the people of Los Angeles ... where devastating fires have broken out in recent days. I pray for all of you."

(Reporting by Jorge Garcia, Rollo Ross, Maria Alejandra Cardona, Joe Brock, Chad Terhune, Matt McKnight, Fred Greaves, Mike Blake, Omar Younis, Sandra Stojanovic and Dawn Chmielewski in Los Angeles; Additional reporting by Brendan O'Brien, Hannah Lang, Rich McKay and David Ljunggren; Writing by Angus MacSwan, editing by Mary Milliken, William Mallard and Alex Richardson)