A burned body has been found inside a central California home after a wildfire believed to have been started by a chainsaw - as an arrested man admitted 'this is going to destroy my life'.

Investigators believe the fire was started by sparks from a chainsaw, authorities said Thursday, August 6.

A 65-year-old man was arrested on Wednesday in connection with the fires that has destroyed seven structures and scorched more than 15 square miles (39 square km) of forest in the Sierra Nevada foothills of Calaveras County.

Herbert Wayne Smith is suspected of "recklessly causing" the blaze on Monday.

It is believed that the fire was cause while "operating equipment capable of producing sparks" without clearing flammable vegetation as required, according to the California Department of Forestry and Fire Protection.

Prosecutors will determine whether to file criminal charges.

Mr Smith told KCRA-TV that he was using a chain saw to cut some scrap wood when it ignited.

"I'm so sorry for everything that's happened," he said.

"This is going to destroy my life, one way or another."

Investigators will determine whether the fire contributed to the death of the person found in a burned home near the town of Angels Camp, the Calaveras County Sheriff’s Office said.

In Washington state, north of California, three major fires continued to burn around the city of Spokane.

Officials said Thursday that cooler temperatures and calmer winds have allowed firefighters to make progress against the fires.

At least 800 buildings have been destroyed, but no deaths have been reported.