The wind-driven Sandy Fire was reported Monday in the hills above Simi Valley, about 50km northwest of Los Angeles.
By Tuesday morning, it had consumed more than five square kilometres of dry brush and destroyed at least one home, according to the Ventura County Fire Department.
The flames were initially pushed by gusts that topped 48km/h, but firefighters were helped by calmer winds overnight, said department spokesperson Andrew Dowd.
"We've made a lot of progress against this fire with those improved weather conditions," Dowd said. Crews hoped to make further progress before winds increased again, he said.
The fire was five per cent contained. The cause is under investigation.
Evacuation orders and warnings were still in place for several neighbourhoods in Simi Valley, a city of more than 125,000 people that was shrouded in smoke as aircraft made water drops.
Meanwhile, firefighters were battling a 60sq/km blaze on Santa Rosa Island, off the Southern California coast. The fire destroyed a cabin and an equipment shed and forced the evacuation of 11 National Park Service employees.
There was no containment as of Tuesday morning.
Santa Rosa, a popular destination for camping and hiking, is home to island foxes, spotted skunks and elephant seals.
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