Two fires in northern California have become the largest in the state's history, fire officials have said.

The fires, known as the Mendocino Complex, are burning around 100 miles north of San Francisco.

They are a few miles apart but started within an hour of each other and are being treated as the same incident.

Since igniting on July 27th, they have devastated 283,800 acres (1,148 Sq km).

Thousands of people have been forced to flee, at least 75 homes have been destroyed and 12 homes have been damaged.

Authorities say the fires are only 30% contained and may not be fully under control for at least another week.

Threat

More than 11,000 structures are listed as being threatened by the Mendocino Complex fires, which have been mostly confined to remote areas.

Scott McLean, a deputy chief with the California Department of Forestry and Fire Protection, described the flames as "extremely fast, extremely aggressive, extremely dangerous."

He added: "Look how big [the fire] got, just in a matter of days.

"Look how fast this Mendocino Complex went up in ranking.

"That doesn't happen. That just doesn't happen."

A Cal Fire plane drops water on a hill above the town of Lucerne in Lake County California. Image: Image: Mark McKenna/Zuma Press/PA Images

Firefighters

More than 14,000 firefighters from a number of US states are fighting various blazes across the country's northwest, almost 4,000 of them dedicated to the Mendocino Complex fires.

Their numbers were bolstered by the arrival of 140 colleagues from Australia and New Zealand on Sunday, 53 of whom will help fight the California fires, while the others head for Oregon.

Mr McLean said: "I can remember a couple of years ago when we saw 10 to 12,000 firefighters in the states of California, Oregon and Washington and never the 14,000 we see now."

Carr Fire

Further north, the Carr Fire has burned through more than 164,000 acres since 23 July and is 47% contained.

The Carr Fire, around 225 miles north of San Francisco, has been blamed on the "mechanical failure of a vehicle," which caused sparks.

It has claimed more than 1,600 buildings, most of them homes, as well as the lives of two firefighters, a power worker and four residents.

Some 38,000 people were forced to leave their homes, although some have recently been allowed to return.

Smoke rises above the hills north of Clear Lake, California, US. Image: Neal Waters/Zuma Press/PA Images

Climate

California has long had problems with fires during the summer but climate change has been blamed for the hotter weather and extremely dry vegetation which have made them more intense and quicker to spread.

Firefighters are unlikely to get any help from the weather either, with temperatures expected to hit 43C (110F) over the next few days.

National Weather Service meteorologist Brian Hurley said: "It's pretty doggone hot and dry, and it's going to stay that way."