Fresh firefighting crews were called in Monday to battle a wildfire chewing westward through coastal mountains of southern Santa Barbara County, one of more than 300 blazes burning in California.

With a heat wave forecast for this week, firefighters on the Gap Fire were trying to take advantage of morning marine fog and a last day of moderate weather, said Stanton Florea, spokesman for the National Forest Service.

Officials called in five ``hot shot'' crews from Arizona and New Mexico, totaling 100 firefighters who will be dropped by helicopter in the Santa Ynez Mountains or hike in to thin brush and slow the fire's progress, Florea said.

``They're going to try to create a line around the western end,'' he said.

Other crews were setting backfires to burn brush so there is no fuel for flames to feed on.

``There's a window we can do some of these firings in,'' Florea said.

The Gap Fire, which started last Tuesday and has spread over about 15 square miles of Los Padres National Forest above the city of Goleta, was 35 percent contained Monday morning, Florea said.

Many residents who fled their homes started to return Monday after most mandatory evacuation orders were lifted in the region about 100 miles northwest of Los Angeles. Some mandatory orders and cautionary warnings remained in effect in scattered mountain communities south of Highway 154 and on the western side of the fire.

Officials did not have the number of homes currently affected. At the height of threats to Goleta-area communities, residents of nearly 2,700 homes were either told to leave or warned to be ready to flee.

A break in summer heat during the weekend allowed firefighters to largely extinguish flames on the southern end, where most of the threatened residential areas were.

But cooler temperatures were forecast to give way to 90-degree heat and low humidity this week.

``There's a heat wave setting in,'' said Jim Andrews, senior meteorologist for Accuweather.com. ``Today is still kind of a transition but it will set in in earnest tomorrow.''

The Forest Service has asked for public help in determining how the fire began. Investigators believe it was human-caused.

Wildfires have burned areas totaling more than 800 square miles and destroyed at least 69 homes throughout California, mainly in the northern part of the state, in the past two weeks. One firefighter died of a heart attack.

Sunday's cooler weather also helped firefighters advance on a 2-week-old blaze that has destroyed 22 homes in Big Sur, on the coast at the northern end of the Los Padres forest.

``The fog held on a little bit stronger than was originally anticipated, which was great for the crews out working on the lines,'' said Sarah Gibson, a spokeswoman for the fire command post.

The improved weather did have some drawbacks. Fog made the takeoff of firefighting aircraft more difficult and hampered efforts to start controlled burns to clear out brush ahead of the advancing wildfire, Gibson said.

The fire, which has charred 117 square miles, was 11 percent contained. Officials said crews were burning out brush between the fire's edge and Big Sur's famed restaurants and hotels and cutting more lines to halt flames creeping down from ridge tops.

``The biggest challenge is whether or not the containment lines that they're building now and continuing to improve are going to hold as the fire approaches,'' said Rolf Larsen, a spokesman for the U.S. Forest Service.

California's siege of fires began with an extraordinary lightning storm in late June. About 1,450 fires have since been contained, but more than 330 still burned out of control Monday morning.

Gov. Arnold Schwarzenegger issued a health warning, urging people affected by smoke to stay inside and limit their physical activity.