Santa Ana winds will continue whipping through Southern California through Thursday, sparking fears that progress made fighting wildfires that have scorched over 40,000 acres and left 28 dead could be reversed and more blazes could break out.

Millions of California residents were placed under a red flag warning through Thursday amid threats of further fires with looming winds in the forecast, according to multiple reports.

The Santa Anas are expected to be most powerful Monday night into Tuesday. Fire services across the region say they are ready.

The Santa Ana winds fanning wildfires that have killed at least 25 people in Southern California and destroyed more than 10,000 houses, businesses and other structures in Greater Los Angeles are flaring up again.

Southern California will continue to face "dangerous fire weather conditions" including strong Santa Ana winds and extremely low humidity through later this week, forecasters said Tuesday.

The winds fueling Southern California wildfires form when a high-pressure system develops over the Great Basin in Utah and Nevada

The strong winds that make wildfires almost impossible to battle and contain picked back up in Southern California Wednesday morning.

Firefighters have slowed the spread of a huge wildfire churning through rugged mountains north of Los Angeles where dangerous winds gained strength again.