(Adds containment estimate, growth of fire)
By Laila Kearney
SAN FRANCISCO, Dec 17 (Reuters) - Hundreds of firefighters battled to gain control of a raging wildfire along central California's scenic Big Sur coastline on Tuesday, hampered by steep and brushy terrain and narrow roads, officials said.
By nightfall, the Pfeiffer fire had blackened 769 acres (311 hectares) and was 20-percent contained, U.S. Forest Service spokeswoman Lynn Olson said. Crews expect to extend containment lines around the perimeter of the fire by Friday, she said.
So far, the fire has destroyed at least 15 dwellings and forced 100 people to flee their homes, fire and county officials said.
The blaze was burning mainly on Pfeiffer Ridge in Big Sur, a mountainous coastal region south of the Monterey Peninsula that reaches into the Los Padres National Forest.
At least 400 firefighters were working to suppress the blaze on Tuesday, but the area's treacherous landscape - with high cliffs and quick-burning brush - hampered their efforts to dig containment lines, Olson said, adding that at least three helicopters were deployed to drop ocean water on the flames.
She said that "extremely steep, brushy terrain and narrow roads," would present a challenge for firefighters on Tuesday night and there was a risk that "dead and dry" trees in the area could fall on them as well as fuel the blaze.
The fire was burning mostly on the ocean side of Highway 1, which runs roughly parallel to the Pacific coast in central California, U.S. Forest Service spokesman Andrew Madsen said. The picturesque highway has remained open despite the fire.
The cause of the fire, which started at around midnight on Sunday near the Big Sur Lodge, about 25 miles (40 km) south of Carmel, was under investigation, Olson said.
Big Sur, famed for its beaches, coastal redwoods and panoramic views of the ocean, is home to a mix of multimillion-dollar houses and eclectic, fringe-living communities. (Reporting by Laila Kearney, Editing by Alex Dobuzinskis, Steve Gorman, Sharon Bernstein and Paul Simao)