Strong winds pushed flames through dry brush in mountains north of Los Angeles on Sunday, and officials warned residents in the wildfire's path to be prepared to leave if it explodes in size again.

Los Angeles County's first major wildfire of the year swiftly grew to nearly 60 square kilometres, one day after it forced the evacuation of at least 1,200 campers, off-roaders and hikers from the Hungry Valley recreation area.

The blaze, dubbed the Post fire, was just two per cent contained Sunday evening. No injuries were reported. The cause was under investigation.

Firefighters working in sweltering conditions and steep terrain raced to douse spot fires that erupted as unpredictable winds blew embers ahead of the flames, said Kenichi Haskett, a section chief for the L.A. County Fire Department. The gusts also hampered efforts by aircraft crews to drop water and fire retardant, he said.

"When it's windy, it just sprays the water everywhere we don't need it. So that's a challenge," Haskett said.

'Have your car fuelled up'

The Southern California fire erupted Saturday afternoon near I-5 in Gorman, about 100 kilometres northwest of Los Angeles. Two structures burned within the evacuated recreation area.

Flames were moving toward Pyramid Lake, a popular destination for boaters that was closed as a precaution on Father's Day. No houses were threatened Sunday, but officials warned residents of Castaic, home to about 19,000 people, that they should prepare to leave if the fire pushes further south.

"If you're in a warning area, be prepared with a 'go bag,' with overnight clothes and your cellphone, your medicines, your glasses. Have your car fuelled up," said Haskett. "Be ready to evacuate."

Low humidity and gusts around 80 km/h were expected throughout the day, and winds could pick up speed after sundown, warned the National Weather Service office for Los Angeles.

About 120 kilometres to the east, the nearly five-square-kilometre Hesperia fire forced road closures and prompted evacuation warnings after it broke out Saturday near mountain communities in San Bernardino County. The blaze was 20 per cent contained Sunday evening.

Meanwhile in Northern California, a small wildfire sparked Sunday prompted evacuation orders and warnings for a sparsely populated area near Lake Sonoma. The so-called Point fire sent up a huge plume of dark smoke as it churned through brush and timber about 130 kilometres north of San Francisco. It was 15 per cent contained.