Firefighters in the US state of California have made more progress in fighting against a massive wildfire, allowing hundreds of evacuees to return to their neighborhoods where they find their homes devastated.

According to state fire officials, around 800 people were allowed to return to their homes in Lake County on Thursday. The number would increase in the next 24 hours.

The so-called Rocky Fire has forced more than 13,000 people to evacuate the burning areas in eight days.

Some of them who return to homes, however, find nothing.

“All of it is gone, it's so surreal," said one of the evacuees, Layna Rivas, after she found her home in Clearlake Oaks had burned to the ground.

"It looked like a bomb went off everywhere, it's all black,"  she explained.

Around 11,000 firefighters backed by helicopters and tankers were struggling to contain the flames across California on Thursday.

California Fire spokesman Daniel Berlant warned that thunderstorms over the next few days have the capacity to produce lightning that could spark more fires. He said large parts of northern California are under a fire weather watch.

He, however, claimed, “Weather conditions are significantly improved from what they were last week."

California Governor Jerry Brown issued a challenge on climate change to Republican presidential candidates ahead of the Thursday night debate.

"It's not a game of politics, it's an existential threat to our well being and our way of life," he said.

"California is burning. What the hell are you going to do about it? And it's not just California, it's extreme weather events throughout the country and the world," he added.

The fire that has been burning since July 29, is only one of 19 major fires burning across California. Cal Fire estimates the Rocky Fire will be contained by August 13.