The fire broke out at about 7:40 a.m. Wednesday next to the Arroyo Seco Day Use Area in the Ventana Wilderness, according to U.S. Forest Service fire information officer Joe Pasinato.
The fire is burning in steep, rugged, oak and chaparral-covered terrain, he said, and firefighters' efforts have been hampered by hot, dry weather.
Nine hand crews, 17 fire engines, two bulldozers, five helicopters, four air tankers and four helicopter tankers, for a total of 250 firefighters, are on scene, Pasinato said.
Responding agencies included the Forest Service, state Department of Forestry and Fire Protection, sheriff's deputies, American Medical Response and the Cachagua Fire Department.
The fire is burning in a northerly direction and flamed up actively overnight, he said, making several upslope runs.
The same area burned in 1977, Pasinato said.
Flames from the fire could threaten structures to the south and east, he said, if the winds pick up. Temperatures were in the mid-90s, humidity 8 to 15 percent, and winds at 3 to 6 mph.
So far one firefighter has suffered a minor eye injury, Pasinato said.
Arroyo Seco Campground was evacuated at the start of the fire and remains closed, he said, as do roads into the area. Flight restrictions on air space over the fire area have also been requested, Pasinato said.
The fire was reported 25 percent contained Thursday afternoon.
No estimate of containment or control has been made, and cause of the fire is under investigation, Pasinato said.
