Firefighters in Southern California reported little headway on Tuesday against two explosive, wind-whipped wildfires that forced the evacuations of tens of thousands of US residents.

Howling Santa Ana winds, blowing hot, dry air from the desert, had put much of the fire-ravaged state under red-flag warnings and helped quickly spread two blazes in Orange County that erupted on Monday.

More than 750 firefighters using 14 helicopters managed to contain only 5 percent of the Silverado fire, which grew overnight to 11,200 acres (4,500 hectares) from 7,200 acres (2,900 hectares) late on Monday, the Orange County Fire Authority (OCFA) said.

Two firefighters were critically injured fighting the blaze, with second- and third-degree burns over much of their bodies, the OCFA said.

The fire in the foothills of the Santa Ana Mountains south of Los Angeles prompted officials to order more than 90,000 residents to leave their homes on Monday in and around the city of Irvine, officials said.

The other Orange County blaze, the Blue Ridge fire, which broke out later near Yorba Linda, also spread quickly overnight, charring 15,200 acres (6,150 hectares) by Tuesday morning, OCFA said.

Wildfires this year have ravaged California, scorching more than 6,400 square miles (16,500 square km) – equivalent to the landmass of the state of Hawaii – since the start of the year, with 31 lives lost and thousands of homes destroyed.