Authorities say the Lake Fire ignited just before 1 p.m. on Campbell Mountain, marked with the letters "R" for Reedley and "S" on the side facing Sanger.
The fire is burning near Lake Avenue and Mountain Way.
Officials said flames are moving through rocky outcroppings and across retardant lines, complicating containment efforts.
The steep terrain is also making firefighting operations more difficult.
Approximately 130 firefighters are assigned to the incident, with several air tankers assisting from above as crews work to slow the fire's spread.
No additional details about containment or potential threats were immediately available.