By Alex Dobuzinskis

LOS ANGELES (Reuters) - Two rapidly growing wildfires burning a few miles apart in parched foothills just northeast of Los Angeles threatened to merge on Tuesday after forcing the evacuation of more than 700 people, officials said.

The blazes came as California and other southwestern U.S. states baked in a heat wave.

The so-called Fish Fire and the Reservoir Fire, which both broke out on Monday in the Angeles National Forest, more than doubled in size overnight and were entirely unconfined, the U.S. Forest Service said in a statement. (http://bit.ly/28Lbe6h)

The Fish Fire, whose cause is under investigation, has grown to 3,000 acres (1,214 hectares) while the Reservoir Fire, which fire officials say was sparked by a car crash, stood at about 2,400 acres (971 hectares), according to figures from the U.S. Forest Service.

"It is a possibility that the two fires would merge," Andrew Mitchell, a spokesman for the team battling the Reservoir Fire, said in a phone interview.

The fires burning more than 20 miles (32 km) northeast of downtown Los Angeles have forced at least 700 people to evacuate, Mitchell said. The communities nearest the flames include the suburban towns of Duarte and Azusa.

Overnight, a flank of the Fish Fire crept down a hillside on the east side of Duarte, lapping at brush just beyond some houses before firefighters extinguished the flames, Los Angeles County Fire Chief John Tripp said at a news conference.

"Our big threat today is still that left side of the fire," Tripp said. "That still is a very uncontrolled flank of the fire."

Officials warned more evacuations could be ordered.

While the two blazes have not yet merged, they are being handled as one incident called the San Gabriel Complex Fire. Over 600 firefighters are battling those blazes fueled by dry brush and chaparral, officials said.

Meanwhile, a half-dozen other wildfires burned across California.

In the coastal part of the state, firefighters have made steady progress in handling the so-called Sherpa Fire, a seven-day old blaze northwest of Santa Barbara that has burned nearly 8,000 acres (3,237 hectares) in an area of ranches and campgrounds. That fire is 70 percent contained, according to tracking website InciWeb.gov.

Two states away, the Dog Head Fire in central New Mexico has charred more than 17,000 acres (6,880 hectares) 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; Editing by Phil Berlowitz and Cynthia Osterman)