Firefighters are battling a 34-acre fire above San Bernardino that prompted the closure of Highway 18 Monday afternoon.

The blaze, dubbed the Peak Fire, was reported around 11 a.m. in the area of Old Waterman Canyon Road, off Highway 18 in the mountains just north of San Bernardino.

The fire was spreading rapidly with about two-dozen homes threatened, according to San Bernardino National Forest officials.

The flames jumped the highway around 11:20 a.m. then quickly grew from an estimated 8 – 10 acres to 30 acres in a little over an hour.

Highway 18 was closed down in both directions at Waterman Avenue and 40th Street, as well as Waterman Avenue and 48th Street. The closure was later extended from Highway 138 down to 40th Street.

More than 200 firefighters and 14 aircraft converged on the fire, dousing the mountainous area with fire retardant.

One firefighter was taken to a hospital due to a heat-related injury, officials said.

“We’re working in really, really tough country — super steep terrain in over 100 degree heat,” San Bernardino County Fire Battalion Chief Mike McClintock said in a video update.

By 1 p.m., fire officials said crews were making good progress battling the blaze, but highway was still expected to be closed “for a significant amount of time.”

It’s unclear what sparked the fast-moving fire.

A California Highway Patrol log listed a report of a BMW that crashed into the side of a mountain and caught fire at Old Waterman Canyon Road, just before the brush fire was reported. Officials did not indicate whether the crash was suspected to be the source of the fire, only saying that the cause of the blaze was under investigation.

The fire ignited as the region was under a heat advisory, with a high of 102 degrees forecast for the area Monday.