"Surrounding areas should be ready to leave if needed. Please don't hesitate to evacuate," the Siskiyou County Sheriff tweeted.
Highway 96 and McKinney Creed Road southwest of the Klamath River were closed to the public, CalFire said.
Yreka resident Larry Castle told the Sacramento Bee newspaper that he and his wife had packed up a few possessions and their three dogs to leave the area for the night, as other fires in recent years had taught them the situation could turn "very, very serious."
Nearly 650 people were working to douse the blaze as of Sunday, the National Wildfire Coordinating Group said.
Firefighting forces were sent from nearby Oregon to help containment efforts, the Oregon State Fire Marshal said, as the Klamath National Forest also deals with the Kelsey Creek Fire.
CalFire said the cause of the fire was still "under investigation."
The US Forest Service said "a heavy smoke inversion" had helped to limit the growth of the fire on Sunday, but also meant that firefighters' aircraft were also "mostly grounded."
Fire crews were working above Fort Jones and west of Yreka "to cut off the fire's progress," the USFS said.
The record-breaking blaze was sparked just days after the year's previous largest fire raged in central California.
The Oak Fire near Yosemite National Park broke out in mid-July and spread rapidly, destroying 41 buildings and forcing thousands to evacuate.
California, which is facing a punishing drought, still has months of fire season ahead of it.
In recent years, California and other parts of the western United States have been ravaged by huge and fast-moving wildfires, driven by a warming climate.
Other parts of the world have also faced intense wildfires this year.
On Sunday, both Portugal and France were battling major forest fires, as temperatures rose sharply in Europe over the weekend.
Also over the weekend, hundreds of firefighters in Germany battled a blaze in the east of the country, with four people injured, authorities said.
Scientists say climate change is making heatwaves around the world more frequent and more intense, increasing the risk of fires.
