More than 20,000 homes and other residences were being evacuated in San Diego County on Tuesday as a fierce California wildfire blackened more than 700 acres, fire and county officials said.


Most of the homes being evacuated were in the city of San Diego, with some in an unincorporated part of the county, according to the San Diego County Emergency website.


The fire, which erupted on Tuesday morning as Southern California baked under temperatures above 38 C, was only five per cent contained by late afternoon, fire officials said.


Sheriffs deputies were advising residents by phone to evacuate ahead of the flames, and an evacuation centre was set up for displaced residents at an area high school.


California officials have kept staffing levels for wildland firefighters at elevated levels since last year because the state is in the midst of its worst drought in decades.