By Laura Zuckerman
SALMON, Idaho, Aug 19 (Reuters) - A wildfire that has forced the evacuation of more than 2,000 homes in central Idaho roared largely unchecked into a 12th day on Monday near the ski resort of Sun Valley, even though fire crews launched a big offensive against the blaze at the weekend.
Firefighters took advantage of calmer winds and higher humidity levels on Sunday to attack the flames aggressively, but still ended the day with containment lines carved around less than 10 percent of the blaze's perimeter, fire officials said.
The lightning-sparked fire has been raging since Aug. 7 near Sun Valley and the adjacent tourist towns of Ketchum and Hailey. It has charred some 101,000 acres (41,000 hectares) of parched sagebrush, grasslands and pine forests in the Sawtooth National Forest.
On Sunday afternoon, local authorities had expected to lift a mandatory evacuation for 200 homes in two neighborhoods north of Hailey. They said those plans were on hold Sunday night as gusty winds threatened to breathe new life into the blaze.
The ever-shifting nature of the so-called Beaver Creek fire, has frustrated fire managers. The blaze has exhibited erratic, even extreme behavior, engulfing whole trees and making unpredictable runs down mountainsides.
Beth Lund, incident commander with the U.S. Forest Service team managing the blaze, nevertheless expressed cautious optimism on Sunday about prospects for gaining ground on the fire.
"I think we're getting to the point where we can start making some progress rather than just be on the defensive," she said. "But when Mother Nature feels she has the upper hand, she keeps it for a while."
More than 1,100 firefighters have been assigned to the blaze, backed up by bulldozers, water-dropping helicopters and airplane tankers carrying fire-retardant chemicals.
The fire has displaced occupants of at least 2,250 homes in posh developments outlying Sun Valley and Ketchum and Hailey immediately to the south.
Authorities have put the value of land and property threatened in the resort region, known as the Wood River Valley, at $8 billion. The area contains the homes of such celebrities as film director Steven Spielberg, actor Tom Hanks and singer and actress Barbra Streisand.
At the Sun Valley Resort, workers on Sunday turned on water cannons usually used to make snow to wet down a mountain whose southeastern face was the scene of a concentrated assault by firefighters.
The blaze has sent towering columns of smoke over an area prized for its scenic views, mountain lakes and a river teeming with wild trout.
Scores of residents and tourists fled over the weekend from Sun Valley and Ketchum, where streets - and many homes - were largely deserted. Residents of the twin towns were advised by Blaine County Sheriff Gene Ramsey to be ready to leave at a moment's notice. (Reporting and writing by Laura Zuckerman; Editing by Steve Gorman and David Brunnstrom)