A massive fire described as the biggest in the history of Los Angeles was raging Saturday, forcing hundreds of people to evacuate their homes.

Mayor Eric Garcetti told reporters the blaze, which broke out Friday and lit up the hills surrounding the northern suburb of Burbank overnight, had already burnt 5,000 acres (2,000 hectares), adding: "In terms of acres involved this is probably the largest fire in L.A. city history."

The blaze has so far destroyed one house and forced the evacuation of nearly 500 homes in various parts of the city and neighboring Glendale.

More than 500 firefighters were mobilized, while 100 others who had been deployed in Houston, Texas to help rescue operations after Hurricane Harvey were asked to return.

The fire broke out on the U.S. Labor Day holiday long weekend and as the western part of the country was facing a summer heatwave.

Authorities said wind was their main concern because it could whip up the flames and allow it to spread quickly in unpredictable directions.

The blaze in thick brush that has not burned in decades was slowly creeping down a rugged hillside on Saturday toward houses, with temperatures in the area approaching 100 degrees Fahrenheit (38 degrees Celsius), the Los Angeles Fire Department said in an alert.

"This fire, which broke out yesterday, we can now say is the largest fire in the history of L.A. city, in terms of its acreage," Garcetti told reporters.

The fire could make air unhealthy to breathe in parts of Los Angeles, the nation's second-largest city, and nearby suburbs, the South Coast Air Quality Management District said in an advisory.

Video posted online by local media showed the fire burning along the 210 Freeway when it broke out on Friday, with smoke hovering over the roadway as cars passed by flames a few dozen feet away. Officials quickly closed a stretch of the freeway.

More than 400 miles (644 km) to the north, the so-called Ponderosa Fire has burned 3,880 acres, or about 1,570 hectares, and destroyed 30 homes in Butte County since it broke out on Tuesday. It prompted authorities to issue evacuation orders earlier this week to residents of some 500 homes.

The blaze was 45 percent contained.

California Governor Jerry Brown issued an emergency declaration on Friday to free up additional resources to battle the Ponderosa blaze.

Wildfires in the U.S. West have burned more than 7.1 million acres (2.9 million hectares) since the beginning of the year, about 50 percent more than during the same time period in 2016, according to the National Interagency Fire Center.

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