California's lightning-sparked wildfires more than doubled in size into some of the largest in state history on Friday, forcing over 175,000 people to flee their homes.

At least six people have died, 43 fire fighters and civilians have been injured, and over 500 homes and other structures destroyed as one fire burned within a mile of the University of California Santa Cruz (UCSC) campus. Firefighting forces were depleted from over 370 fires and so far only 45 of 375 out-of-state fire crews requested by California had arrived, said a spokeswoman for wildfire authority Cal Fire.

The state has been hit by its worst dry-lightning storms in nearly two decades. Over 11,000 strikes have sent fire racing through grassland and forest parched by record-breaking heat, burning 771,000 acres (1,205 square miles) since Aug.15, an area larger than the state of Rhode Island.

"We simply haven't seen anything like this in many, many years," Governor Gavin Newsom said, appealing for additional federal support.

Most of the fires are in the San Francisco Bay Area, with a complex of blazes east of Palo Alto and another in wine country south of Sacramento now the seventh and tenth largest in state history, respectively, according to Cal Fire.

One fire approached the campus of UCSC, which is located in hills around three miles (4.8 km) northwest of coastal city Santa Cruz's boardwalk.

"The fire continues to advance, and much of what will happen next depends on weather conditions such as wind direction and speed," UCSC Chancellor Cynthia Larive wrote in a tweet, after ordering the evacuation of the campus.

"We're still understaffed for a fire of this size," said Daniel Potter, a Cal Fire spokesman, in reference to the Santa Cruz blaze, which has forced 64,000 people to evacuate from vulnerable towns where houses are built amongst the trees.

One of those is Felton, a community of 4,000, where some stayed behind to defend homes after fire torched houses and California's oldest state park to the north, a Reuters photographer said.

Around 1,000 firefighters in the area fought multiple fires blown in shifting directions by erratic winds as propane tanks and ammunition stores exploded in homes, he said.

In the North Bay Area, four people died in a cluster of fires that have destroyed over 480 homes and structures in wine counties such as Napa, Solano and Sonoma, Cal Fire reported.

A utility crewman died on Wednesday while on duty helping clear electrical hazards for first-responders at the same fire, dubbed the LNU Complex. Earlier that day, the pilot of a firefighting helicopter contracted by the state was killed in a crash in Fresno County.

The largest fire, known as the SCU Complex, east of Palo Alto, more than doubled in size from Thursday to around 230,000 acres or 359 square miles, an area approaching the size of New York City.