SAN DIEGO -- Some evacuation orders were lifted early Friday in an area near the fiercest of several wildfires in Southern California's San Diego County, as crews building containment lines around the blazes hoped cooler temperatures will help them make further progress.

County officials said residents of two neighbourhoods were allowed to return home. A flare-up Thursday in a north San Diego suburb prompted more than 18,000 new evacuation notices as flames raced through tinder-dry brush on hillsides. That fire was 10 per cent contained Friday morning.

Ten major fires have destroyed at least eight houses, an 18-unit condominium complex and two businesses since Tuesday. The hardest-hit areas were in San Marcos and Carlsbad, a suburb of 110,000 people that lifted evacuation orders late Thursday.

Firefighters found a badly burned body Thursday in a transient camp in Carlsbad -- the first apparent fatality -- and a Camp Pendleton Fire Department firefighter was treated for heat exhaustion.

To the north crews battled two fires at Camp Pendleton, one of the largest military bases in the U.S., including one that broke out Thursday and exploded overnight, base officials told KGTV. It was 10 per cent contained. The second fire, sparked Wednesday on the eastern outskirts of the Marine base, scorched nearly 10 square miles of dry brush. That blaze was 15 per cent contained.

This week's unseasonably early wildfires have driven tens of thousands from their homes and shut down schools and amusement parks, including Legoland, which reopened Thursday. Flames have caused more than $20 million in damage.

Firefighters who have worked in temperatures sometimes topping 100 degrees Fahrenheit (38 Celsius) this week were expected to get relief on Friday. The forecast called for temperatures to peak around 90 and lighter winds. A bigger cool-down was forecast for the weekend.

It could take months to find the causes of the blazes concentrated in the northern San Diego and its northern suburbs, from the coast to areas up to 24 kilometres inland. San Diego County Sheriff Bill Gore said arson will be among the many possibilities that investigators will look at.

Crews along with engines and aircraft were ready to spring into action when the first fire ignited Tuesday after being sent to the region in anticipation of a heat wave and gusty winds.

The positioning of crews was among several steps fire officials say they have been fine-tuning since 2003 when the San Diego area experienced one of the worst infernos in California's history. Communications between firefighting agencies has improved, residents are notified more quickly when to evacuate, and more aircraft are available to dump water on fast-moving flames.

San Diego County had some of the strongest winds Wednesday, with gusts reaching up to 80 kph, which may have set conditions for fires to be easily ignited, just as they were in 2003. The 2003 fire scorched more than1,131 sq. kilometres, nearly 3,000 buildings -- including more than 2,000 homes -- and killed 15 people before being contained.

The tragedy led to California creating one of the world's most robust firefighting efforts, which resulted in the smooth evacuation of thousands this week and crews able to save hundreds of homes from being consumed by the fast-moving wildfires, said Battalion Chief Nick Schuler of the California Department of Forestry and Fire Protection.