A heat dome over the US south-west has translated into extreme heat warnings from coast to coast, which continue to affect more than 110 million people.
Temperature records could be broken in as many as 38 cities.
In Las Vegas, the intense heatwave was threatening to break or tie the city's record high of 117F (47.2C).
It comes as soaring temperatures are also hitting southern Europe and Canada is battling the worst season of wildfires in its history.
Scientists have long warned that climate change linked to human activities will lead to an increase in the severity and frequency of extreme weather events.
Elsewhere in the south-western US, hundreds of firefighters have been battling brush fires in blistering heat and low humidity on the outskirts of Los Angeles.
Temperatures in Death Valley in California hit 128F (53.9C) on Sunday, according to the National Weather Service (NWS). It is the site of the hottest temperature ever reliably recorded on Earth: 134F (56.7C).
The usually crowded streets of Las Vegas were considerably emptier than normal on Sunday, with security guards guarding the fountains of upscale casinos and hotels to prevent people from jumping in.