Mandatory evacuations have been ordered and a highway has been closed as dozens of firefighters worked to contain a fast-moving blaze burning in the mountains near Azusa on Thursday afternoon.

The Dam Fire erupted in the 9500 block of North San Gabriel Canyon Road, near Morris Dam in the Angeles National Forest, the Los Angeles County Fire Department tweeted shortly before 1:45 p.m.

The fire rapidly grew to 120 acres within an hour of it first being reported, according a tweet from the Angeles National Forest.

More than two hours after igniting, the blaze had charred 240 acres and was 0% contained. But crews appeared to be making significant progress in controlling the blaze’s spread, with aerial units making quick work of hotspots that erupted, Sky5 video showed.

Flames initially spread uphill in heavy brush at a “rapid rate of spread” west of Highway 39, officials said. The highway has been shut down until at least Sunday, according to Azusa police.

At least 100 firefighters have been dispatched to battle the blaze from the ground, while more than a dozen aircraft are attacking flames from the air.

Around 4 p.m., mandatory evacuations were ordered in the area north of Morris Dam to Crystal Lake, with evacuees asked to leave via East Fork to Glendora Mountain Road.

Earlier, campers in the West Fork area of the forest were evacuated as a precautionary measure, according to the Glendora Police Department.

Residents of the Mountain Cove community have also been urged to prepare, the Azusa Police Department said.

While plumes of heavy smoke were visible from nearby community, police emphasized that no evacuations have been ordered in Azusa, Glendora, San Dimas or Claremont.