Firefighters battling history-making wildfires in Northern California say the weather is not in their favor as they struggle to bring more than 600 separate blazes under control.

Hot weather, unpredictable winds, and possible lightning strikes that could set more fires are making an incredibly difficult job even harder.

The U.S. National Weather Service forecasters posted a Red Flag Warning for Sunday and Monday for the San Francisco Bay area north along the coast.

These warnings mean that warm temperatures, stronger winds and lower humdity are expected to combine to produce an increased risk of fire danger.

"There's a lot of potential for things to really go crazy out there," California fire chief Mark Brunton said, noting that the winds can blow from any direction.

President Donald Trump declared a major disaster area for Northern California, making the state eligible for federal funds to help those who have lost their homes and property.

State officials are calling the fires the second-largest clusters of wildfires in recorded California history.

As of Sunday, about 450,000 hectares have been torched. More than 1,000 homes and buildings have been incinerated. Thousands have fled their homes. At least five fire-related deaths have been reported.

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There are reports that a number of ancient redwood trees have been destroyed.

Soot, smoke and haze are clouding the skies over other parts of the state, creating a health hazard.

Experts blame the latest fires on storms that brought lots of lightning but no rain. The extreme heat that baked California last week, including a 54 degree Celsius (130 Fahrenheit) reading in Death Valley, is aggravating the fire risk. (VOA)