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A flare-up from this month’s devastating Holy Fire has burned dozens of acres in the Santa Ana mountains and appeared to threaten a communications tower at Santiago Peak after jumping containment lines Monday morning.

The Cleveland National Forest reported the new activity shortly after 10 a.m., tweeting a photo of flames and smoke near Santiago Peak.

By 11 a.m., the flare-up had jumped outside of containment lines and had grown to at least 40 acres, officials said.

Though it grew slowly, by 4:15 p.m. the flare-up had burned 150 acres total and was 10 percent contained, according to the National Forest.

However, officials noted that the numbers would likely be adjusted after the blaze is more accurately mapped.

The flare-up is expected to push full containment on the Holy Fire back several weeks, firefighters said.

Earlier in the afternoon, the Orange County Fire Authority had said crews were making good progress after accessing the flames, which were burning in a remote area deep within the forest.

The majority of the flames were burning back into the Holy Fire burn area, with a small portion scorching through previously unburnt fuels, U.S. Forest Service spokesman JakeCarothers told KTLA.

Ten aircraft, including air tankers and helicopters, were battling the flames from overhead, authorities said. Ground resources — roughly 300 personnel, nine fire engines and one water tender — were working to increase containment around Santiago Peak, which houses vital communication infrastructure.

Smoke is expected to be visible in the surrounding communities, but there was no immediate threat to Orange County.

The closest homes are about two miles away, at the bottom of the slope, Carothers said.

He noted the fire was burning uphill, toward a communications facility that is utilized by many different departments and agencies for emergencies.

“That’s the value at risk at this time,” Carothers said, noting it was the main focus of the firefight.

Firefighters are working to protect the communications towers and infrastructure, according the Fire Authority.

“There’s a lot of area up there… green islands within the fire that are still burning,” he said. “It’s not really active, but situations like this happen, where all of a sudden, we have something else outside of the containment line.”

Authorities told KTLA the flames are believed to be a flare-up from the Holy Fire, which scorched nearly 23,000 acres in the Cleveland National Forest across Orange and Riverside counties before it was fully contained.

The blaze erupted in Holy Jim Canyon, near Trabuco Canyon, on Aug. 6.

Santiago Peak is the highest point in Orange County, reaching an elevation of nearly 6,000 feet.

Temperatures in the area early Monday evening reached 78 degrees, with humidity around 52 percent and light winds blowing out of the west with speeds of up to 5 mph, according to the National Weather Service.

Carothers said firefighters were hoping the winds would reverse direction closer to nightfall, pushing the flames back on themselves.

KTLA’s Jennifer Thang contributed to this story.