A baking heatwave that has gripped the US south-west for three days has spread east to Iowa and Missouri, while punishing the hardest-hit areas with record high temperatures that have strained power systems.

On Thursday (local time), the National Weather Service issued excessive heat warnings for much of the south-west, including Arizona, southern Nevada, much of California and southern Utah.

Heat advisories were issued for parts of the Central Plains, including Kansas, Nebraska, Iowa and Missouri.

“It’s a pretty big impact with respect to where the record heat is,” NWS meteorologist Bob Oravec said from the Weather Prediction Centre in Maryland.

For the past three days, a high-pressure system has been parked over the south-west, a region used to temperatures of about 38 degrees between now and September.

On Tuesday, Salt Lake City, Utah, tied its all-time high temperature since record-keeping began in 1894, at 42 degrees. Phoenix also equalled a record, at 46 degrees, while Needles in the Mojave Desert near the border of Nevada and Arizona, hit 121 degrees, also tying a daily record.

Forecasters say more records could fall on Thursday, where by late-afternoon temperatures could hit 47 degrees in Phoenix, and 45 degrees in Las Vegas.

California’s Death Valley National Park, typically one of the hottest spots in the world, recorded 54 degrees on Wednesday.

The brutal heat has triggered a surge in electricity demand as residents cranked up air-conditioners and fans. America’s two most populous states, Texas and California, have pleaded for residents to conserve energy.

On Thursday, California’s grid operator issued a “Flex Alert”, telling people to turn off unneeded appliances and office equipment and set their thermostats higher.

California and Texas have both imposed rolling blackouts in recent months – California due to the heat in August 2020 and Texas in February 2021 after a deep freeze that left millions without heat.

Elsewhere, the National Weather Service has issued red flag warnings across the south-west to warn of fire dangers. Major blazes are already burning across the region, although most were more than 50 per cent contained as of Thursday afternoon.

A cold front is expected to bring relief to the midsection of the country on Sunday as temperatures in the south-west slowly moderate as well.

-with AAP