Although the 22,885-acre Holy Fire was officially 91 percent contained, video from a stationary camera atop Santiago Peak near Trabuco Canyon showed active flames and massive plumes of smoke shortly after 10 a.m.
By the afternoon, the new flareup was estimated at 150 acres, but firefighters said they were making good progress containing the flames.
There were no immediate reports of new injuries or structural damage, and no evacuation orders were issued.
"The fire is currently no threat because it is in un-burned fuel surrounded by already burned vegetation," the Orange County Fire Authority tweeted.
The Cleveland National Forest agency dispatched resources to the scene, and the OCFA sent a helicopter and a hand crew to assist with what it described as a "flare-up."
Holy Fire: Flames reignite near Santiago Peak in Cleveland National Forest, prompting renewed response
Five air tankers and four helicopters were assigned to the incident, the agency said, adding that "ground resources will be utilized when it is safe to do so."