The fire was 80 percent contained by about 8:45 p.m. tonight, but Battalion Chief Jim Crawford said full containment would likely not be reached until Friday morning due to the steep and rocky terrain.
The blaze was reported shortly before 4 p.m. in the area of Niles Canyon and Palomares roads and approximately 300 firefighters from Cal Fire and local agencies were combating the flames this afternoon, Cal Fire Capt. Jerry Agenbroad said.
Four firefighters have suffered minor or moderate injuries, one from heat and the three others due to falling rocks and debris.
Crawford said firefighters will monitor the blaze overnight and continue fighting the fire at sunrise Friday.
"The fire's not moving anywhere but we can't really make access until morning," Crawford said.
California Highway Patrol Officer Peter Van Eckhardt said Palomares Road is expected to be closed between state Highway 84 and Interstate Highway 580 due to the fire until Friday morning.
No structures have been threatened by the flames.