A fast-moving wildfire in California continues to grow in size, firefighters say, as they struggle through sweltering temperatures.

The Oak Fire has now burned 15,603 acres of land and is still "zero per cent contained", California's fire department said on Sunday night.

But it added that the fire activity is not as extreme as previous days, and firefighters are making progress.

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

A further 3,271 structures - both homes and businesses - are under threat, the California Department of Forestry and Fire Protection (Cal Fire) reported.

At a community meeting on Sunday night, an official said they hoped the blaze, which started on Friday, would start to be contained "very soon".

A state of emergency was declared in Mariposa County on Saturday, and will allow the state to access some federal assistance to tackle the fires.

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 100F (38C) on Sunday, and are expected to stay high 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.