Residents of about 200 were forced to evacuate Saturday after a brush fire erupted in the San Gabriel Mountain foothills east of Los Angeles, authorities said.

The fire was sparked by a gardener working the backyard, said Jennifer McLain, a city spokeswoman. Flames spread to the hillside behind the residence, scorching 150 acres.

The fire was not immediately threatening homes, but authorities ordered evacuations as a precaution, fire Chief Chris Donovan told KABC-TV.

The blaze was 50 percent contained and residents were allowed to return home early Sunday.

Donovan said dry, thick brush and the hilly terrain were making the fire difficult to contain.

Helicopters dropped water and firefighters called in two fixed-wing aircraft.

"We got a lot of fire resources protecting homes, but we're nowhere near out of the woods," Donovan said. "We have a lot of work ahead of us."

As the Monrovia fire burned, a four-acre brush fire ignited across town near Interstate 405, forcing the closure of southbound lanes for at least an hour.