Twin wildfires fueled by intense heat in the US state of California have forced residents of two cities to flee their homes.

The fires roared through brush surrounding suburban neighborhoods near the San Gabriel Mountains in northern Los Angeles on Tuesday, making residents from almost 1,000 homes to leave the area.

The San Gabriel Complex fires in Azusa and Duarte have burnt more than 8 square miles and firefighters have not been able to contain the spreading flames so far.

“We had fire on the hillsides above those structures,” said Chief John Tripp, of the Los Angeles County Fire Department. “Most of the night, that fire crept down and came to the bottom of the slope.”

“We made tremendous progress on the east side of Duarte. Our big threat today is the west flank. There's a fire line that goes from the bottom of the slope all the way to the top of the mountain.”

The fire first broke out around 11:00 am Monday on Highway 39 near the Morris Reservoir dam north of Azusa, and the second one erupted in areas near Duarte.

“I heard the helicopters all night,” said Timothy Blyston, who works in Azusa. “I can see the smoke still."

“I saw flames and a line of fire hugging the mountains,” said Sebastian Alonzo, who works in Duarte.

More than 300 firefighters were striving to contain the fires and more firefighters were expected to join the battle from Santa Barbara and San Diego.

There were fears that the two fires could merge and form a huge inferno.

Also, the so-called Sherpa Fire in Santa Barbara County, which started last Wednesday fed by parched land and high winds, was 82 percent contained Tuesday at 7,969 acres (3224 hectares), with full containment expected Thursday, officials said.

Fire department officials say that such fires are similar to those that erupt in the fall.

“We're here at the beginning of June and we're seeing very active fires, very similar to behavior that we would typically see in the fall,” said Daniel Berlant, a spokesman for the Cal-Fire firefighting agency.

Meanwhile, a record-setting heat wave has claimed at least four lives in the neighboring state of Arizona, in eastern part of which a fire on the Fort Apache Indian Reservation expanded to over 50 square miles.