A brush fire of unknown origin blackened scores of open acres in a hilly section of Camp Pendleton today, prompting road closures and evacuations of office buildings on the Marine Corps station.

The blaze erupted shortly before 1:30 p.m. off Vandegrift Road and 17th Street, in the southeastern reaches of the USMC installation, said Staff Sgt. Jesse Lora, a spokesman for the base.

As military and civilian ground and airborne crews began working to halt the spread of the flames, authorities cleared everyone out of structures near the burn area and shut several stretches of road on the base.

By early evening, the fire had scorched about 100 acres, Lora said.

No structural damage or injuries were reported.