Emergency managers in Los Angeles apologized Friday after false evacuation alerts sparked panic in a city on edge over an unprecedented wildfire disaster.

Millions of mobile phones blared on Thursday afternoon and again on Friday morning with automated warnings urging people to be ready to flee.

"This is an emergency message from the Los Angeles County Fire Department. An EVACUATION WARNING has been issued in your area," said the message, which was widely disseminated on Thursday, including to areas well away from the danger zones.

"Remain vigilant of any threats and be ready to evacuate. Gather loved ones, pets and supplies."

Huge fires in the Pacific Palisades neighborhood of Los Angeles and around Altadena have consumed 35,000 acres, razing thousands of buildings and killing 10 people.

For many Angelenos, the alert system was the first they knew of the blazes and the need to escape.

Some 153,000 people are currently under mandatory evacuation orders across the region.

A correction was sent 20 minutes later, explaining the alert had only applied to the new Kenneth Fire that was exploding to the north of the city.

Nevertheless, a similar erroneous message was sent out around 4 am Friday.

Kevin McGowan, the director of the Los Angeles County Office of Emergency Management, said the automated errors had caused "frustration, anger (and) fear."

"I can't express enough how sorry I am," he told reporters.

McGowan said he was working with specialists to find out what had gone wrong and why so many people were sent messages that did not apply to them.

"I implore everyone to not disable the messages on your phone," he said.

"This is extremely frustrating, painful and scary, but these alert tools have saved lives during these emergencies."

Los Angeles County Supervisor Lindsey Horvath said the error was "unacceptable."

"I want to express my deep frustration with the alert system that is causing confusion and additional panic for our communities at this time of extreme crisis," she said.

"Whatever the cause, it is unacceptable."

Chris Sheach, assistant professor in disaster management at Paul Smith's College, said automated warning systems are always susceptible to "kinks and bugs," particularly as they are rarely used at scale, but still remain vital in reducing the death toll during disasters.

"Likely it was a coding error," that caused warnings to reach unintended recipients in the wrong area codes, he told AFP.

"The benefits far outweigh the risks. If this had been the Palisades in 1975... the only way they could evacuate people was fire trucks driving street by street with a megaphone."

Sheach said it was important for authorities to be up front about the mistakes, so that citizens do not ignore or disable future alerts.

"Millions of these messages go out every year across the country, across the globe, to communities that are appropriate, safe, and on time," he said.

"Because of those misses, it takes a little extra work to build the trust."