LOS ANGELES (Reuters) - A fast-moving wildfire that has already blackened some 11,000 acres (4,450 hectares) of drought-parched land northwest of Los Angeles was threatening some 1,000 structures on Saturday, fire officials said.

The so-called "Sand Fire" broke out shortly after 2 p.m. on Friday afternoon and spread quickly near Santa Clarita, about 40 miles (65 km) northwest of the city, forcing the evacuation of some 300 people.

It has nearly tripled in size in less than a day and was only 10 percent contained.

"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."

One firefighter sustained a minor injury but as of Saturday morning no structures had been destroyed, officials said. No deaths have been reported.

Some 300 firefighters in temperatures expected to hit highs of 106 degrees Fahrenheit (41.1°C) were battling the flames, 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.

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.

(Reporting by Jon Herskovitz in Austin, Texas and Dan Whitcomb in Los Angeles; Editing by Bernard Orr and Alistair Bell)