The blaze, dubbed the Blue Fire, broke out around 11:50 a.m. off the northbound 15 Freeway near Cleghorn Road, according to the San Bernardino National Forest.
Officials said the flames had the potential to burn 30-40 acres, and by 3 p.m., the fire was 40% contained. Fire crews were expected to remain on scene into the evening.
No evacuations or injuries have been reported, but some lane closures are in effect on the freeway.
The cause of the fire is under investigation.
DEVELOPING: We will add more details to this report as they become available.