A Los Angeles County fire helicopter makes a night drop while battling the so-called Fish Fire above Azusa, California, June 20, 2016. REUTERS/Gene Blevins

A fast-moving wildfire that has forced hundreds of people to flee from their homes in the drought-parched foothills northwest of Los Angeles had blackened some 20,000 acres by Saturday night and was threatening homes and commercial structures, fire officials said.

The so-called Sand Fire broke out shortly after 2 p.m. on Friday and spread quickly near Santa Clarita, about 40 miles (65 km) northwest of the city, prompting evacuation orders covering about 1,500 homes.

By late Saturday night the Southern California blaze was about 20 percent contained, casting a pall of thick black smoke over much of Los Angeles and threatening about 100 commercial structures.

Los Angeles County Sheriff’s homicide detectives were investigating circumstances surrounding a man found dead in an evacuation area. Media reported a resident was found dead in a burned car, citing a neighbor who said the man's house has burned down.

Residents earlier posted pictures on social media of the sun blotted out by the towering plumes and the South Coast Air Quality Management District issued a smoke advisory, warning of unhealthy air conditions in the region.

Officials said one firefighter sustained a minor injury, and that structures had been destroyed or damaged in the Bear Divide and Sand Canyon areas.

"Because this is the fifth year of an ongoing drought we have a lot dry vegetation," Los Angeles County Fire Chief Daryl Osby told a news conference.

"Some of these fuels, they haven't burned in decades. This fire has increased to about 11,000 acres just overnight."

Some 900 firefighters were battling the flames in temperatures exceeding 100 degrees Fahrenheit (41 Celsius), with the aid of 28 helicopters and eight fixed-winged aircraft. But fire managers said crews were struggling in very rugged terrain as they tried to defend homes in the community of Little Tujunga and stop the spreading blaze that is burning through chaparral and brush.

Evacuation shelters have been set up for residents in the area and about 10 roads have been closed due to the fire. A number of roads in and out of foothill communities were shut down.

The fire is one of a series this summer that have hit the drought-stricken state, where dried grass and bush land as well as high temperatures have helped fuel the blazes.