By Alex Dobuzinskis

LOS ANGELES (Reuters) - Firefighters were gaining ground on Friday against a wildfire burning in a Southern California mountain pass that has forced tens of thousands of residents to flee their homes and destroyed about 100 houses, officials said.

The Blue Cut fire, named for a narrow gorge near its origin in the Cajon Pass about 75 miles (120 km) northeast of Los Angeles, has blackened 37,000 acres (14,973 hectares) of drought-parched heavy brush and chaparral after breaking out on Tuesday.

The blaze has destroyed 96 single-family homes and 213 outbuildings, according to a preliminary assessment from teams in the field, fire information officer Lyn Sieliet said by telephone. Officials previously said dozens of structures were gutted, without providing exact figures.

Officials said firefighters were able to carve containment lines around 26 percent of the blaze as of Friday morning - up from 4 percent a day earlier - in dry, hot and windy weather conditions and treacherous terrain.

The intensely burning blaze, which has produced cyclone-like whirls of flame, continued to threaten some 34,500 homes and other structures in communities including the ski resort town of Wrightwood, fire officials said.

More than 80,000 residents were told to evacuate their homes on Tuesday. Since then, some people have been allowed to return home, Sieliet said, but she could not say how many.

While many residents opted to stay put, the San Bernardino County Sheriff's Office said deputies arrested three people suspected of attempting to loot from the abandoned homes of evacuees.

Transit authorities on Thursday reopened Interstate 15, the primary traffic route between greater Los Angeles and Las Vegas, Nevada, after it was closed for two days by the fast-moving blaze.

The Blue Cut fire is one of nearly 30 major blazes reported to have scorched some hundreds of square miles in eight Western states this week, in the midst of a wildfire season stoked by prolonged drought and unusually hot weather, according to the National Interagency Fire Center in Boise, Idaho.

(Additional reporting by Curtis Skinner in San Francisco; editing by Janet Lawrence and Cynthia Osterman)