FIRE officials say more than 82,000 people have been told to evacuate from mountain communities threatened by a powerful wildfire in Southern California.

San Bernardino County fire spokesman Eric Sherwin says the evacuation orders were given to about 34,500 homes.

Sherwin says some structures have already burned, but it hasn’t yet been determined whether any of them were homes.

#BlueCutFire in Cajon Pass (San Bernardino Co) is 9000 acres, 0% contained, 82,000 residents under mandatory evac. pic.twitter.com/n44GyY2leC — CAL FIRE PIO Berlant (@CALFIRE_PIO) August 17, 2016

The 22-square-kilometre fire near Cajon Pass has seen massive growth in the eight hours since it broke out on Tuesday morning.

The blaze, 96-kilometres east of Los Angeles, has shut down a section of Interstate 15, the main route between Southern California and Las Vegas.

As that fire surged, a major blaze north of San Francisco was fading and some 4,000 people in the town of Clearlake were allowed to return home.

The wildfires were the latest in a weeks-long stretch of heat and drought-driven fires across California that raged well before the official start of wildfire season in early autumn.