A wildfire near Yosemite National Park in California is spreading quickly, threatening thousands of homes.
The California Department of Forestry and Fire Protection (Cal Fire) described it as being "zero per cent under control" on Saturday morning.
"Explosive fire behaviour is challenging firefighters", Cal Fire reported on Saturday.
Central and North Eastern regions of the US are also sweltering under extreme heat.
Temperatures of 38C (100.4 F) are forecast in Washington DC and Dallas, with New York only slightly cooler.
An official in Oklahoma - where the temperature is expected to reach 41C on Sunday - said heat was the main cause of weather-related deaths in the US.
Dubbed the Oak Fire, the blaze in California started on Friday afternoon local time and quickly exploded in size. Thousands of people have been told to evacuate.
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.