LOS ANGELES - Three fast-moving wildfires continued to burn in Southern California Tuesday as millions of residents in the region are still under an excessive heat warning.

The largest one, dubbed the Line Fire, had scorched at least 26,000 acres (about 105 square km) with only 5 percent containment as of Tuesday morning in San Bernardino County, forcing mandatory evacuation orders for thousands of people.

There are currently 65,600 structures threatened, including 9,200 structures under evacuation orders and 56,400 structures under evacuation warnings, according to the California Department of Forestry and Fire Protection (Cal Fire).

"The north and east sides of the Line Fire continue to burn with the greatest intensity. Today will be mostly sunny over the fire area, and the smoke impacts are expected to grow as the day progresses," said Cal Fire in an update, adding that firefighters are creating control lines, protecting structures and the community, making contingency plans, and maximizing the use of aircraft when conditions are not too smoky.

The fire, which broke out on Thursday, grew exponentially in size over the weekend due to soaring temperatures and critically dry vegetation amid a record-setting heat wave in Southern California.

California Governor Gavin Newsom declared on Saturday a state of emergency in San Bernardino County to help ensure the availability of vital resources to suppress the fire. He announced on Monday the activation of the California National Guard to assist nearly 2,000 firefighters in battling the massive wildfire.

Another wildfire, called the Bridge Fire, broke out on Sunday in the Angeles National Forest in Los Angeles County, nearly 100 km west of the Line Fire. The blaze grew to nearly 3,000 acres (about 12.1 square km) with zero containment as of Tuesday morning.

A new blaze, dubbed the Airport fire, started Monday afternoon in Trabuco Canyon in Orange County. The fire charred more than 8,500 acres (about 34.4 square km) overnight with zero percent containment, prompting evacuations.

The fire spread quickly in hazardous weather conditions, leading to the evacuation of more than 1,400 homes and triggering responses by more than 1,000 firefighters, reported local KABC television station, adding that two firefighters were treated for heat-related injuries and one civilian was hospitalized for smoke inhalation.

Southern California is still facing dangerously hot conditions on Tuesday, and an excessive heat warning will stay in effect until Tuesday night, according to US National Weather Service Los Angeles.

The massive wildfires have generated unhealthy levels of smoke in Southern California. The South Coast Air Quality Management District issued a wildfire smoke advisory, valid until Tuesday night, for a large part of the region.

"Smoke from the Line, Bridge, and Airport fires is expected to cause PM2.5 levels to reach at least the Unhealthy for Sensitive Groups," said the agency.