Despite light vegetation to fuel it when it started, a brush fire in the Castaic area Saturday grew from an initial three acres to more than 3,000 acres in four hours with 20 to 30 homes ordered evacuated, authorities said.

Hundreds of firefighters from Los Angeles and Ventura counties were on the scene and the fire was 10 percent contained by lines of cleared vegetation, officials said. Two helicopters were scheduled to make water drops through the night.

The fire — dubbed the Charlie Fire due to Charlie Canyon Road, where the blaze first was sighted — was reported at 2:40 p.m., according to Los Angeles County Fire Department Inspector Joey Marron.

The fire covered a few acres at the top of a ridge when firefighters arrived and sized it up at 2:44 p.m., Marron said.

They quickly called for a third-alarm response of 120 firefighters, aided by three helicopters and two super scooper fixed-wing aircraft, he said.

A strike team made entirely of 22 Los Angeles city firefighters was on scene along with a helicopter, spokesman Brian Humphrey said. A second strike team was being assembled with LAFD and allied agencies represented.

By 6 p.m. the Santa Clarita Valley sheriff’s station reported Lake Hughes Road was closed from Ridge Route to Dry Gulch and that San Francisquito Canyon Road was closed and evacuated from Lowridge Place North to Camp 14. About 100 homes were threatened and 20 to 30 homes along Francisquito Canyon Road were ordered evacuated and the Castaic Sports Complex at 31230 Castaic Road was designated as the evacuation center.

No structures were damaged and no injuries were reported, officials said.

The rough terrain that was making access difficult was cited as a reason for the blaze’s spread.

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