More than 50,000 people were under evacuation orders or warnings on Wednesday as a huge and fast-moving wildfire swept through the rugged mountains north of Los Angeles.

But fire officials said a rapid ground and air assault was giving them the upper hand.

The Hughes Fire broke out in the late morning and within six hours charred about 15 square miles (39 square kilometres) of trees and brush near Lake Castaic, a popular recreation area about 40 miles (64 kilometres) from the devastating Eaton and Palisades fires that are burning for a third week.

Though the region was under a red flag warning, winds were not as fast as they had been when those fires broke out, allowing for firefighting aircraft to dump tens of thousands of gallons of fire retardant.

“The situation that we’re in today is very different from the situation we were in 16 days ago,” Los Angeles County Fire Chief Anthony Marrone said on Wednesday evening.

There were no reported homes or other structures burned.

“This fire had a robust response today, and as you can see behind us, the responders are doing great work to try to contain this fire," Joe Tyler, director of Cal Fire, said. “Certainly, we are not out of the woods yet.”

More than 31,000 people have been ordered to evacuate, and another 23,000 are under evacuation warnings, LA County Sheriff Robert Luna said.

Parts of Interstate 5 that had been closed reopened on Wednesday evening. (AP)