The wild fires in Northern California sparked by lightning on Friday has exploded in size. Some of the states have seen the biggest fire in the state's history, forcing thousands to flee and killing six.

Five deaths involved fires burning in wine country north of San Francisco, and the other was a helicopter pilot who crashed while dropping water on a blaze in Fresno County. 

California has recorded nearly 11,000 strikes in 72 hours, mostly in northern and central California.

More than 12,000 firefighters aided by helicopters and air tankers are battling wildfires throughout California. Three groups of fires, burning north, east and south of San Francisco have together scorched 780 square miles (2,020 square kilometers), and destroyed more than 500 structures.

More than 140,000 people are under evacuation orders.

The majority of blazes started less than a week ago, and comes during a heat wave that has seen temperatures top 100 degrees Fahrenheit. They are taxing the state's firefighting capacity but assistance from throughout the country was beginning to arrive, with 10 states sending fire crews, engines and aircraft to help, Gov. Gavin Newsom said.

The number of personnel assigned to the sprawling LNU Complex – a cluster of blazes burning in the heart of wine country north of San Francisco – doubled to more than 1,000 firefighters Friday, he said.

"We have more people but it's not enough. We have more air support but it's still not enough and that's why we need support from our federal partners," Newsom said.

The SCU Lightning Complex fire burning east of San Francisco started Tuesday and the slightly smaller LNU Lightning Complex burning in wine country that was sparked a day earlier already have become among the 10 largest wildfires in state history. Firefighters had only contained a small portion of the wildfires by Friday afternoon.

There are 560 fires burning in the state, many small and remote but there are about two dozen major fires, mainly in Northern California. 

Tens of thousands of homes were threatened by flames that drove through dense and bone-dry trees and brush. Some fires doubled in size within 24 hours, fire officials said.

(With input from agencies)