A wildfire descends Saturday night on a home in La Canada Flintridge, 32 kilometres outside downtown Los Angeles. (Philip Scott Andrews/Associated Press)A wildfire in the mountains above Los Angeles surged Sunday in every direction, going in a single day from a modest threat to a danger to some 12,000 homes.

The blaze nearly tripled in size as temperatures approached 40 C on the weekend, killing two firefighters Sunday and injuring three people. It also destroyed at least 18 structures on Sunday, including three homes, and forced the evacuation of 1,000 homes.

Mandatory evacuations were in effect for neighbourhoods in Altadena, Glendale, Pasadena, La Crescenta and Big Tujunga Canyon.

The flames crept down the slopes of the San Gabriel Mountains despite mild winds blowing predominantly in the other direction.

'Perfect storm' conditions

"Today what happened is what I call the perfect storm of fuels, weather, and topography coming together," said Capt. Mike Dietrich, the incident commander for the U.S. Forest Service.

"Essentially the fire burned at will; it went where it wanted to when it wanted to."

Dietrich said he had never seen a fire grow so quickly without powerful Santa Ana winds to push it.

At least three homes deep in the Angeles National Forest were destroyed, and firefighters were searching for others, Dietrich said.

Twelve evacuation centres were set up at schools and community centres in the area.

The fire, which began on Wednesday, was the largest and most dangerous of several burning around southern and central California and in Yosemite National Park.

The fire spread especially to the north and west, bringing new concerns for the areas near Acton and Santa Clarita.

The fire scorched more than 164 square kilometres of dry forest. It was only five per cent contained.

Injured airlifted to hospitals

Two firefighters died Sunday afternoon when their vehicle rolled off a mountainside near Mt. Gleason in the Angeles National Forest, officials said.

At least three people, including two from Big Tujunga Canyon who tried to ride out the fire in a backyard hot tub, were burned in the evacuation areas and airlifted to local hospitals, Dietrich said.

One of the two from the hot tub remained in hospital in stable condition. The condition of the third person wasn't disclosed. The Associated Press earlier reported that five people were hurt.

Aircrews waged a fierce battle against the southeast corner of the fire, burning dangerously close to canyon homes.

The fire was burning in steep wooded hills next to NASA's Jet Propulsion Laboratory in northern Pasadena.

In La Vina, a gated community of luxury homes in the Altadena area, a small group of residents stood at the end of a cul-de-sac on the lip of a canyon and watched aircraft battle flames trying to cross the ridge on the far side.

At one point, the flying contingent of relatively small propeller-driven tankers gave way to the sight of a giant DC-10 jumbo jet unleashing a rain of red retardant.

"We see a drop, we give a big cheer," said Gary Blackwood, who works on telescope technology at the propulsion laboratory.

"We've watched it now for two days hop one ridge at a time and now it's like we're the next ridge."

A major goal was to keep the fire from spreading up Mount Wilson, where many of the region's broadcast and communications antennas and the historic Mount Wilson Observatory are located, officials said.

But officials said Sunday evening the flames were a few hours from reaching Mount Wilson. Television stations said if the antennas burn, broadcast signals will be affected but satellite and cable transmissions should not be.

With files from The Associated Press