The Canyon Fire ignited on Thursday afternoon and has grown to more than 19.7 square kilometres, according to the Ventura County Fire Department.
At least 400 firefighters were battling the blaze along with several planes and helicopters. It remained uncontained and was spreading east into Los Angeles County, officials said.
The fire is burning just south of Lake Piru, a reservoir located in the Los Padres National Forest. It's close by Lake Castaic, a popular recreation area burned by the Hughes Fire in January.
Sunny, hot and dry conditions were expected in the area where the Canyon Fire was burning on Friday, with the daytime high 38C and minimum humidity in the mid-teens, according to the National Weather Service.
In LA County, around 2700 residents evacuated with 700 structures under an evacuation order, officials said. Another 14,000 residents and 5000 structures were covered by an evacuation warning.
Areas within the Val Verde zone had been reduced from an order to a warning.
The evacuation zones in nearby Ventura County were relatively unpopulated, Ventura County Fire Department spokesperson Andrew Dowd said. Fifty-six people were evacuated from the Lake Piru recreation area.
Dowd called the blaze a "very dynamic situation" caused by hot, dry weather, steep and rugged terrain and dry fuel.
LA County Supervisor Kathryn Barger urged residents to evacuate.
"Extreme heat and low humidity in our north county have created dangerous conditions where flames can spread with alarming speed," Barger said in a statement. "If first responders tell you to leave, go without hesitation."
The new blaze comes as a massive wildfire in central California became the state's largest blaze of the year, threatening hundreds of homes and burning out of control in the Los Padres National Forest.
The Gifford Fire had spread to 402 sq km by Thursday night with 15 per cent containment. It grew out of at least four smaller fires that erupted on August 1 along State Route 166 and has injured at least four people. The causes of the fires are under investigation.
Wildfire risk will be elevated through the weekend across much of inland California as a heat wave gripping the area intensifies. August and September are typically the most dangerous months for wildfires in the US state.
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