As the SCU Lightning Complex of fires nears 375,000 acres, Cal Fire reported early Saturday that firefighters were making slow progress and containment was at 40 percent.
The fire was 374,471 acres as of Saturday morning, an increase of 1,500 acres over the previous day, according to Cal Fire.
Five people have been injured in the blaze, three of them first responders, Cal Fire said.
The fire had destroyed 33 structures as of Saturday morning — five more than Friday — and is threatening another 20,065 more.
Many large trees have burned, and weakened by the fire, continue to fall and block roadways, with their stumps retaining extreme heat, Cal Fire said.
The fires were sparked by lightning Aug. 16 and are burning in Santa Clara, Alameda, Contra Costa, San Joaquin, Stanislaus, Merced and San Benito counties. The blazes have merged into two branches.
Nearly 2,000 personnel from out of state have come in to assist Cal Fire fight the massive fires.