The governor of California has declared a state of emergency after wildfires forced thousands of people to flee their homes in the north of the state.

Governor Jerry Brown said the fires had destroyed buildings in the Napa and Lake counties and threatened hundreds of others.

More than 1,300 people fled Middletown, north of San Francisco, as their homes were consumed by the flames.

Four firefighters who were badly burned are receiving treatment in hospital.

The fires across northern California are being blamed on high temperatures and years of drought.

The state spent $212m (£137m) fighting the flames in July alone, California's forestry and fire protection department spokesman Daniel Berlant told the AFP news agency.

He said more than 275 homes and other buildings had been destroyed and the Red Cross is opening emergency shelters for evacuated residents.

One blaze, the Valley Fire, which started on Saturday in Lake County, is said to have burned 40,000 acres (16,190 hectares).

It was reported to have reached the centre of the small town of Middletown on Sunday. Its 1,500 residents had already been ordered to evacuate.

The fire spread quickly and witnesses saw flames reach up to 200ft (60m) in the air, according to local news reports.

Further east, in Amador and Calaveras counties, around 4,000 firefighters are battling the Butte Fire, which broke out on Wednesday

That blaze has so far destroyed around 65,000 acres (26,300 hectares) along with 86 homes and 51 outbuildings. It is only 15% contained and threatens more than 6,000 other buildings, officials say.

Further south, beyond Fresno, firefighters have been tackling the largest of the blazes, the so-called Rough Fire, which has claimed 128,800 acres (52,000 hectares) since it began in late July.

Nearly 3,000 firefighters are tackling that blaze, which is now said to be 29% contained.

They have evacuated the Kings Canyon National Park and working to protect the park's famous grove of Giant Sequoia trees.