Thousands of residents have fled their homes in northern California as some 5,000 firefighters tackle wildfires.

One blaze, the Valley Fire, which started on Saturday in Lake County, north of San Francisco, has already burned 40,000 acres (16,190 hectares).

Four firefighters were injured and had to be airlifted to hospital, where they are said to be in a stable condition.

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.

The small town of Middletown looked to be under threat from the Valley Fire on Sunday. Its 1,500 residents had already been ordered to evacuate.

Local media reported that the much of the town centre was ablaze.

The fire spread quickly and witnesses saw flames reach up to 200ft (60m) in the air, the local Press Democrat newspaper 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.