By Alex Dobuzinskis

LOS ANGELES (Reuters) - Two rapidly growing wildfires burning a few miles apart through drought-parched foothills northeast of Los Angeles prompted the evacuation of nearly 800 homes on Tuesday, as a heat wave continued to bake much of the U.S. Southwest for a third straight day.

Abnormally high temperatures climbing into the high 90s and triple digits across much of the region helped stoke wildfires from the coastal hills outside Santa Barbara to desert brush near the Mexican border, and several other Western states.

Fires also were menacing populated areas in New Mexico, Arizona and along the Colorado-Wyoming border.

The greatest immediate threat was posed by a pair of fires that erupted near each other on Monday near the Angeles National Forest and doubled in size overnight as they roared unchecked through foothills and canyons above the communities of Azusa and Duarte northeast of Los Angeles.

Those fires, jointly referred to by authorities as the San Gabriel Complex, had devoured some 5,400 acres combined, according to latest estimates on Tuesday evening.

About 770 homes were placed under evacuation on Tuesday, no property losses or serious injuries were reported, and fire officials said late in the day that flames appeared to be creeping mainly in the direction of uninhabited wilderness areas.

More than 1,000 firefighters were battling the conflagration, part of which was ignited by a car crash, officials said.

Along the coast to the north, firefighters made steady progress in corralling the so-called Sherpa Fire, a seven-day old blaze northwest of Santa Barbara that has burned nearly 8,000 acres in an area of ranches and campgrounds. That fire was 70 percent contained as of Tuesday, authorities said.

To the south, firefighters battled flames roaring through dry brush and chaparral near the Mexican border for a third day, prompting authorities to expand evacuation orders around the desert community of Potrero with 1,000 structures under threat.

That fire, about 50 miles southeast of San Diego, had charred more than 6,020 acres and was 10 percent contained on Tuesday evening, California fire officials reported.

To the east, Arizona Governor Doug Ducey issued a state of emergency for Navajo County where the so-called Cedar Fire had charred over 40,000 acres. That blaze was 22 percent contained.

Along the Colorado-Wyoming state line, a wildfire in the Routt National Forest burned uncontrolled on Tuesday, prompting authorities to issue mandatory evacuation orders. The so-called Beaver Creek fire, believed to be human caused, has blackened nearly 800 acres and threatens some 30 homes and structures.

The Dog Head Fire in central New Mexico has charred nearly 18,000 acres and was 46 percent contained after destroying 24 homes and 21 minor structures soon after it broke out last week.

(Additional reporting by Laila Kearney in New York, Steve Gorman in Los Angeles, Curtis Skinner in San Francisco and Keith Coffman in Denver; Editing by Phil Berlowitz and Cynthia Osterman)