The fire reportedly started on Saturday around 7:30 p.m. in the upper Mill Creek drainage area of the forest, south of Nacimiento Fergusson Road.
Prewitt Ridge, Alms Ridge and Mill Creek area landowners were advised to evacuate on Sunday. The fire was about two miles from the nearest home, forest spokeswoman Kathleen Good said.
Good said the fire crossed the north side of the Nacimiento Fergusson Road and is also on the west and east sides of South Coast Ridge Road.
All campgrounds along the two roads have been evacuated.
As precaution, residents along state Highway 1 in the Limekiln, Hermitage and Harlan property areas were advised to get ready for a possible evacuation, she said.
No injuries have been reported.
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MORE: ABC7's Bay Area wildfire resource guide and safety tips
MAP: Google Reference Map of California Fires
(From the Governor's Office of Emergency Services).