LOS ANGELES, Sept. 10 (Xinhua) -- Firefighters have substantially expanded the encirclement of the arson fire in Northern Los Angeles, which is now 71 percent contained, compared to 62 percent one day ago, fire commanders said on Thursday.

    Around 3,650 firefighters remain assigned to the blaze, which still threatens nearly 4,000 structures, according to the National Fire Service (NFS), which is leading the firefighting efforts. The east flank of the fire remains uncontrolled, but construction of fire containment lines around the San Gabriel Wilderness Area "is going well," the NFS said in a statement.

    Meanwhile, fire crews successfully carried out a controlled burn to prevent the 16-day fire from threatening Mount Wilson, the site of a historic Observatory and a number of broadcast communications towers.

    The controlled burn had been postponed for three straight days before an improvement in wind conditions allowed firefighters to carry it out overnight, amid cooler temperatures.

    "Firefighters burned out vegetation approximately half way between Mount Wilson and Cogswell reservoir in extremely thick vegetation. The burn was successful," the NFS said.

    The controlled burn removed brush from areas where anticipated Santa Ana winds could have sparked spot fires, according to the NFS statement.

    The fire has scorched more than 250 square miles (400 square kilometers) since it broke out on Aug. 26. As of Thursday morning, fighting the largest fire in Los Angeles County history has cost nearly 78 million dollars, said NFS spokesman Tom De Bellis.

    Also on Thursday, work began to assess damage caused to homes and businesses affected by the fire, according to a statement from Los Angeles County's Office of Emergency Management. The assessment effort involves personnel from the Small Business Administration, California Emergency Management Agency and the county's Office of Emergency Management, according to the county.

    Authorities said the fire was sparked by an arsonist. The state has offered a 100,000-dollar reward for information leading to an arrest and conviction, and the Los Angeles County Board of Supervisors also authorized a 50,000-dollar reward.

    The fire has claimed the lives of two firefighters, injured 13 firefighters and one support worker, and destroyed 78 homes, mostly cabins, as well as two commercial properties and 86 outbuildings.