At least four people, including a German man, have died from the blistering heat that has gripped the western United States and sparked wildfires and evacuations across the region, authorities said Monday.

Three hikers and a mountain biker succumbed to the heatwave in Arizona on Sunday, where record-breaking temperatures were reported in some areas.

Larry Subervi, spokesman for the Phoenix fire department, said one of the victims was an experienced 28-year-old female mountain biker who had embarked on a two-and-a-half-hour ride in the Phoenix area with enough water but got overwhelmed by the heat.

Three other people reportedly died in the Tucson area, including a 57-year-old German national -- identified as Stefan Guenster -- who was hiking with two other fellow Germans on the Ventana Trail early Sunday

The Pima County Sheriff's Department, located in Tucson, said one of the men was able to make it down the trail to get help.

Rescuers found Guenster dead near the trail and the third man, identified as 33-year-old Marcus Turowski, is still missing.

The other victim was identified as a 54-year-old woman who went for a walk along a path known as The Loop and was found dead after her husband reported her missing.

"We have a heatwave every year, but we are close to our all-time record in 1990 of 122 degrees Fahrenheit (50 Celsius)," Subervi told AFP.

He said temperatures on Sunday had soared to 120 degrees and the crushing heat continued on Monday, setting records in many areas of Arizona and southern California. Temperatures were expected to ease from Tuesday.

The heatwave has fed wildfires in California, New Mexico and Arizona, forcing evacuations in some areas.

Two fast-moving brush fires tore through 3,500 acres (1,416 hectares) in the San Gabriel Mountains east of Los Angeles on Monday, prompting some 600 households to be evacuated, officials said.

- 'Steep terrain, heavy vegetation' -

Fire officials said hundreds of firefighters were battling the flames in the foothills above Azusa and Duarte, aided by water-dropping helicopters.

"The fire is burning in very, very steep terrain with very heavy vegetation," said Robert Garcia, the fire chief at Angeles National Forest, referring to the initial blaze -- dubbed the Reservoir Fire -- north of Azusa.

He said that fire was sparked by a fatal car crash.

The second blaze -- dubbed the Fish Fire -- erupted about four miles away in Duarte and quickly roared into the foothills, triggering evacuations and threatening some homes before moving in a different direction.

Fire officials said residents of nearby communities should be ready to evacuate in case the flames gain ground during the evening.

"There are a lot of open flanks of the fire that tonight, if we get (winds), we could have more evacuations," said John Tripp, deputy chief of the Los Angeles County fire department.

Fire officials said a canyon separated the two fires but there were fears they could merge into a huge inferno.

Further north, in the Santa Barbara area of California, some 2,000 firefighters for several days have been battling the so-called Sherpa fire that has already burned nearly 8,000 acres (3,200 hectares) and prompted the evacuation of 140 households.

In New Mexico, the Dog Head fire about 30 miles (48 kilometers) south of Albuquerque has also damaged two dozen homes and scorched nearly 18,000 acres, officials said.

They said that as of Monday, only nine percent of the fire had been contained.

Another fire caused by lightning near Cibola National Forest, in New Mexico, has eaten up more than 36,000 acres and was 30 percent contained.

The fire, which started on May 21, was expected to be contained by late July.