Gusty winds failed to return Thursday morning in San Diego County wildfire areas and authorities said it was a window of opportunity to make further gains against flames that burned homes and drove tens of thousands from their homes.

Emergency officials said a significant number of firefighting aircraft had become available, including four air tankers and 22 military helicopters.

Ten of the military helicopters were being used to battle a blaze that grew 24 square kilometres on the Marine Corps' Camp Pendleton. Despite its growth, the fire was 20 per cent contained and was no longer considered a threat to communities.

A U.S. military helicopter drops water to help stop an approaching wildfire in San Marcos, Calif., on May 15, 2014. (Mike Blake/Reuters)

Twelve other military helicopters were available to the county, where the biggest concern was a 3-square-kilometre blaze at the city of San Marcos. That fire was 5 per cent contained and thousands of people remained evacuated, but officials told a news conference they were beginning to assess repopulating areas.

Fires began erupting Tuesday amid high heat, extremely low humidity and gusty Santa Ana winds. By Wednesday, nine fires were burning.

Asked about the possibility of arson, county Sheriff Bill Gore said he wouldn't prejudge the investigations. He noted that sparks from vehicles can easily ignite brush in such dry conditions.

The wildfires forced tens of thousands of people to evacuate their homes and shut down schools and amusement parks, including Legoland.

Firefighters contended with temperatures approaching 38 C and gusty winds as they tried to contain flames fueled by brush and trees left brittle by drought.

Extremely high temperatures were again expected Friday, but winds were expected to be light to moderate, with localized gusts. Forecasters predicted a return of the normal sea breeze on the western edge of the county later in the day.

Efforts were focusing on San Marcos, a university city where hundreds of new evacuation orders were issued early Thursday.

3 San Diego County homes gutted

More than 20,000 evacuation notices were sent to residents Wednesday, and a California State University campus with nearly 10,000 students in the middle of final exams was shut down at least through Thursday. Graduation ceremonies were cancelled.

San Diego County officials said that the blaze had destroyed three homes.

The blaze in the coastal city of Carlsbad, about 48 kilometres north of San Diego, was the most destructive of the fires so far.

Many schools across the county were closed Thursday. Officials expected some wouldn't reopen until next week.

Gov. Jerry Brown declared a state of emergency for San Diego County, which would free up special resources and funding for the firefight, and state fire officials were creating a central command centre for the blazes.

Drought conditions have made fire danger extremely high throughout much of California. Officials have encouraged residents in fire-prone areas to prepare evacuation plans and clear brush from near their homes.