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Crews were battling a 100-acre fire off the 15 Freeway near the Cajon Pass Friday amid 110-degree heat, officials said.

The Box Fire began about 9:30 a.m. when a semi truck fire spread to brush along the freeway near Kenwood Avenue, according to the California Highway Patrol.

The blaze was about 15 percent contained, Cal Fire tweeted shortly before 2 p.m.

At least two firefighters were transported for heat exhaustion, officials said. Meanwhile, two lanes of the freeway were closed during the firefight, authorities said.

Cal Fire, San Bernardino National Forest and the San Bernardino County Fire Department responded to the blaze.

At one point, the fire was moving at a “moderate rate of spread,” according to forest officials.

The fire comes during the peak of a dangerous heat wave that is expected to bake much of Southern California. National Weather Service officials warned residents of possible fire conditions and said to use extreme caution with potential ignition sources. It was expected to reach a high of 108 degrees in the area of the fire Friday afternoon.

“Right now it’s hot,” said Cathey Mattingly, an information officer for Cal Fire. “Firefighters are working really hard, so the main focus is gonna be safety, hydration, personal care, making sure that they’re paying attention to their own conditions while they’re fighting this fire.”

A San Bernardino County fire official told KTLA structures were threatened in Devore Heights and units were on scene for structure protection. Later, fire officials said the structure threat had been “mitigated.” No evacuations have been ordered.

The National Forest has activated a call center for the blaze. Residents can call 909-383-5688 for information.