Los Angeles County firefighters battle a brush fire near Pacific Coast Highway and Mulholland Highway in Malibu Thursday, Feb. 25, 2016.

MALIBU, Calif. (KABC) -- A female inmate firefighter was critically injured after being struck in the head by a boulder while battling a 7-acre brush fire in Malibu Thursday morning, authorities said.Crews were seen performing life-saving measures until a Los Angeles County Fire helicopter arrived and airlifted the firefighter to Ronald Reagan UCLA Medical Center, where her condition was later upgraded from critical to serious.The inmate firefighter was a member of Fire Camp 13, an all-female camp based in Malibu, and working with Caltrans.The blaze erupted around 3 a.m. in steep, inaccessible terrain near Pacific Coast Highway and Mulholland Highway, a region of rugged slopes and peaks with widely scattered homes and youth camps.L.A. County fire officials initially said the blaze had burned 20 acres but later revised the acreage. As of 11 a.m., the blaze was 75 percent contained.Sheriff's officials said they helped some nearby residents evacuate from their homes, but the exact number was unknown. Around 120 kids at nearby Camp JCA Shalom and Camp Bloomfield were on standby to evacuate, but no evacuations were ordered.Light winds were assisting helicopter crews making water drops over the fire.Mulholland Highway is closed between Little Sycamore Canyon Road and Pacific Coast Highway. No injuries have been reported.The cause of the fire remains under investigation.