Out-of-control wildfires surrounding Los Angeles are continuing to expand, forcing families to evacuate and blanketing the sky with choking smoke.

Three separate blazes have erupted in the mountains to the east of the United States’s second-biggest city, fuelled by a punishing heatwave and fanned by gusting winds on Wednesday.

Thousands of firefighters, with support from the US National Guard, were battling to slow the spread of fires tearing through tinder-dry brush.

There have been no reports of deaths or serious injuries, but tens of thousands of homes and businesses are being threatened by the looming flames.

The fast-moving Airport Fire in Orange County has consumed more than 9,000 acres (3,600 hectares) since it was started accidentally on Monday afternoon by workers operating heavy equipment.

Orange County Fire Authority Captain Steve Concialdi said the fire was continuing to grow and bearing down on Santiago Peak, home to radio and television broadcast towers used by local media outlets, as well as federal and local government agencies.

Concialdi said crews were working to protect the towers on the peak but had to withdraw from the area because the flames became too intense.

Aeroplanes could be seen dumping red fire retardant chemicals on hillsides above a well-to-do neighbourhood, as helicopters dropped water on the flames.

To the northeast of Los Angeles, the Line Fire has now consumed nearly 28,000 acres (11,300 hectares) in San Bernardino County.

Resort communities popular with tourists around Lake Arrowhead and Big Bear in the San Bernardino mountains east of Los Angeles were being told to evacuate, and several major access roads were blocked.

Firefighters were battling the blaze from the air as ground crews sought to establish containment lines, trying to create breaks in the vegetation. But gusting winds and the hot, dry weather was making conditions difficult.

A similar situation was unfolding about 60 kilometres (40 miles) away in the community of Wrightwood, population 4,500, in the San Gabriel Mountains northeast of Los Angeles. The Bridge Fire, which grew tenfold in a day and had burned 194sq km (75sq miles) as of early Wednesday, and is now the largest of three major wildfires burning in Southern California.

Wildfires are a natural part of the wilderness cycle and are not unusual in California and other parts of the US West at this time of year.

After two relatively mild fire years, 2024 is shaping up to have a significant impact.

Two very wet winters generated abundant growth of vegetation, which has now dried out after a long, hot summer, leaving behind lots of fuel.