A plane drops retardant while battling a wildfire near Oroville, California. Picture: Noah Berger/AP

Washington - Firefighters in California on Monday battled several wildfires that have forced thousands of residents to flee and prompted a mobilization of the state's National Guard.

About 4 000 people in an area about 100 kilometres north of the capital Sacramento were under an evacuation order despite a fire there being 35 per cent contained, according to the state fire protection agency.

The fire has burned nearly 23 square kilometres in the grassy foothills in the Sierra Nevada mountains, injuring four firefighters and destroying at least 17 structures.

It is one of 14 fires in California. The state has deployed about 5,000 firefighters and Governor Jerry Brown has declared a state of emergency.

The California National Guard - part of the US army reserve forces - said in a new release that it was in the process of mobilizing to assist civilian firefighters.

Fires broke out amid a heatwave over the weekend that made conditions for fighting the fires even worse. Slightly cooler weather, forecast for the coming days, is expected to bring some relief.

About 3 500 people remain evacuated because of two fires in Santa Barbara County. The largest is 15 per cent contained but is threatening more than 130 homes.

Crews also made progress on wildfires in Colorado, Arizona and New Mexico, according to ABC News.

In Colorado a wildfire temporarily forced the evacuation of hundreds of people near the resort town of Breckenridge. Efforts to battle it were winding down on Monday.

A wildfire destroyed three homes in a rural Arizona community over the weekend, while in New Mexico firefighters were mopping up a wildfire in the mountains overlooking Albuquerque.