The fire is burning in steep, mountainous terrain on the Los Coyotes Indian Reservation that is largely inaccessible by foot so firefighters were being shuttled by helicopters to the fire lines, Cal Fire spokesman Tom Piranio said.
More than 1,800 firefighters are battling the blaze supported by eight air tankers.
Smoke is visible in the nearby town of Borrego Springs but the fire is about 10 miles away and does not threaten the town, Piranio said.
Temperatures were forecast Monday to be 85 to 90 degrees and four firefighters have already been treated at a hospital for heat-related injuries since the fire began at 10:40 p.m. Thursday, The San Diego Union-Tribune reported.
The cause of the fire, which has already cost $4.7 million to fight, is being investigated, officials said.