Washington DC - President Joe Biden said Thursday the wildfires ravaging Los Angeles were the worst in California's history, as he pledged extra federal funds and resources to help the state.

"This is the most widespread, devastating fire in California's history," Biden said as he convened a special meeting of senior administration officials at the White House.

He said people in Los Angeles were "living through a nightmare" and hailed as "heroes" the firefighters whom he said had been running into the flames to battle the blazes.

The outgoing president canceled a trip to Rome scheduled for Thursday to stay in Washington and coordinate the government response to the fires, which have killed at least five people.

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He said the federal government would cover 100% of costs of dealing with the disaster for the first 180 days at the request of California's Democratic governor, Gavin Newsom.

Biden said he was surging 400 federal firefighters and 30 firefighting planes and helicopters to Los Angeles, while the Pentagon will send eight large planes and 500 wildfire clearance personnel.

Vice President and former presidential candidate Kamala Harris, who hails from California, described the fires as "apocalyptic."

She also highlighted what she said were cases of insurance companies canceling coverage for families who lost property in the fire.

Biden meanwhile sought to debunk claims pushed by incoming president Donald Trump that there was a water shortage that had left firefighters struggling to put out the inferno.

Biden said the problem lay with power outages – after utility companies cut electricity amid fears that faulty power lines could spark more fires – that took water pumps offline.

Trump, who will be inaugurated for his second term on January 20, has used the fires to launch political attacks on Biden and Newsom.