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A wildfire exploded in size in the mountains east of San Bernardino on Friday, singeing 1,000 acres and prompting evacuations as it threatened homes in the Forest Falls area, officials said.

Fire crews were working in record-high temperatures as they battled the blaze, which was being dubbed the Valley Fire. The flames broke out around 1:40 p.m. and were still 0 percent contained late Friday night.

The fire had quickly expanded in the evening, jumping from 400 acres in size to 1,000 acres in just 20 minutes shortly before 8 p.m., according to San Bernardino National Forest officials.

It was burning in steep mountainous terrain, and the flames were difficult to access, officials said.

Residents in Forest Falls Valley — where the flames are impinging on about 500 homes — were under a mandatory evacuation order, police said.

An evacuation center was established at Inland Leaders Charter School at 12375 California St. in Yucaipa. Pets and other animals can be taken to the Devore Animal Shelter at 19777 Shelter Way in San Bernardino, officials said.

Trails and recreation sites in the national forest area were evacuate and would remain shuttered until further notice.

There’s believed to be a “large potential” for growth of the fire into the San Gorgonio Wilderness, forest officials said.

About 500 firefighting personnel from multiple agencies were battling the flames, authorities said. The area of Highway 38 between Lake Williams near Big Bear and Angelus Oaks was shut down, officials said at about 8 p.m.

The cause of the fire was under investigation.

It was one of several fires that broke out across the state amid sweltering temperatures on Friday, including one that burned at least 400 acres and several homes in San Diego County. Earlier in the day, another brush fire singed about 100 acres near the Cajon Pass in San Bernardino County.

And another blaze broke out Friday night in Goleta, north of Santa Barbara, prompting evacuations as it moved quickly through the hills and threatened several homes, authorities said. Videos posted by authorities showed at least one structure burning.