The toll from the devastating wildfire in the US state of California has risen to 21 even as around 8,000 firefighters continued to struggle to bring the raging blaze under control, officials said.

The flames have destroyed some 3,500 structures and consumed nearly 170,000 acres, according to the latest figures from Cal Fire, the state government agency that coordinates the response to wildfires, Efe news reported on Wednesday.

Most of the 22 active blazes are in Sonoma and Napa counties, the heart of California’s wine region.

The largest of the outbreaks, the Atlas Fire, is only 3 per cent under control, while the 11 fatalities attributed to the Tubbs Fire make it the 6th-deadliest blaze in California history.

The fires have forced at least 50,000 people from their homes, the Los Angeles Times said, citing official estimates.

“Make no mistake, this is a serious, critical, catastrophic event,” Cal Fire Director Ken Pimlott said at a press conference.

US President Donald Trump issued a federal disaster declaration on Tuesday and California officials said that they were requesting additional assistance from the military and from neighbouring states, who have already contributed 170 fire engines to the effort.

Sonoma County Sheriff Rob Giordano said Wednesday that his office has confirmed 11 deaths related to the fires and that he expects the number to grow once authorities can reach homes destroyed by the flames.

The sheriff’s office has been able to locate 110 of the 670 country residents reported missing in the disaster, Giordano said.

The first fire broke out late Sunday night in Napa County and spread quickly.