As of 4 p.m., the so-called Jim Fire swelled to 400 acres.
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No structures were immediately threatened, according to the Orange County Fire Authority, which was assisting in the firefight along with Cal Fire.
The United States Forest Service said the blaze began about 11:20 a.m. and was spreading at a moderate rate in Holy Jim Canyon area.
The containment was 0% contained as of 4 p.m.
The initial resources deployed against the flames included eight fire engines, two hand crews, two air tankers and two helicopters, officials said.
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One tanker was seen dumping a payload of bright red Phos-Chek on a burning hillside.
The cause of the fire was unknown. Temperatures in the area were in the low 80s Wednesday afternoon, with a humidity of 10%, according to the National Weather Service.
A prescribed burn that had been previously scheduled near Long Canyon Road was canceled at around the time of the Jim Fire's ignition, according to the Forest Service.
For information about the fire, the public can call the Cleveland National Forest's Trabuco Ranger District at 951-736-1811.