A state of emergency has been declared in part of California as the largest active wildfire in the US rapidly spreads near Yosemite National Park.

The Oak Fire has quickly grown since starting on Friday, and firefighters have struggled to get the blaze under control.

"Explosive fire behaviour is challenging firefighters", California's fire department said.

More than 6,000 people have been evacuated and 10 homes destroyed.

The state of emergency, which has been declared in Mariposa County, will allow the state to access some federal assistance to tackle the fires.

Around 400 firefighters and four helicopters have been deployed to fight the blaze. Natasha Fouts, a spokeswoman for CalFire, said officials don't expect to contain the fire until next week.

Locals described evacuating their homes with the flames approaching and one shoeless elderly man attempting to flee the blaze on Friday was rescued by firefighters after crashing his car into a ditch.

The scale of the evacuation means it is proving challenging to find accommodation for all those seeking shelter.

"We're full with evacuees," Alyssa Wildt, a local hotel owner, told the LA Times.

"Today we have a bunch of fire crews looking for rooms," Ms Wildt added. "We don't have them. I believe most of town is full."

The scale of the blaze marks an ominous start to California's wildfire season and officials have warned that a combination of drought, climate change, and overgrown vegetation are posing significant challenges and increasing the chances of the fire spreading rapidly.

Much of the United States is sweltering through a heatwave and heat advisory warnings are in effect in more than a dozen states. Temperatures in Mariposa County hit 96F (35.5C) on Saturday, and are expected to stay at that level for the next few days.

Yosemite National Park, which lies to the north-east of where Oak Fire is burning, is home to some of the largest and oldest sequoia trees in the world. The redwoods were threatened by another blaze earlier this month, but firefighters managed to save them.

The US Forest Service said on Friday it is taking emergency measures to protect the trees, including removing low-lying vegetation that can fuel fires and other measures.

Climate change increases the risk of the hot, dry weather that is likely to fuel wildfires.

The world has already warmed by about 1.1C since the industrial era began and temperatures will keep rising unless governments around the world make steep cuts to emissions.

Several European governments have struggled to contain dozens of separate wildfires which have ripped across the continent in recent weeks. (BBC)

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