Iran Press/ America: Nearly a third of Americans - about 113 million people - are currently under heat advisories, from Florida to California and up to Washington state.
The country's National Weather Service (NWS) has urged people not to underestimate the risk to life, BBC reported.
On Saturday, an all-time high of 118F (48C) was recorded in Phoenix, Arizona.
It means temperatures have hit 110F (43C) for 16 days running, itself almost a record.
Mobile clinics there have reported treating homeless people suffering from third-degree burns.
Meanwhile, Death Valley in California - one of the hottest places in the world - is forecast to reach 129F (54C), nearing the hottest temperatures ever reliably recorded on Earth.
The NWS has said that local records could also be set on Sunday in the San Joaquin Valley, Mojave Desert, and Great Basin regions.
Its Saturday-evening update said the temperatures would "pose a health risk and are potentially deadly to anyone without effective cooling and/or adequate hydration".
About 700 people are estimated to die each year from heat-related causes in the US, according to the Centers for Disease Control and Prevention (CDC).
In neighboring Canada, officials say wildfires stoked by above-average temperatures - which have covered parts of the US in smoke - have now burned nearly 10 million hectares (25 million acres) of land.
The temperatures in America's south-west are the result of an upper level ridge of high pressure, which typically brings with it warmer temperatures, the NWS said earlier, adding that the heatwave was "one of the strongest" systems of its kind to hit the region.
Las Vegas, Nevada, may also match its all-time high of 117F (47C) in the next few days.
Weather officials there warned locals who thought they could handle the temperatures that this was "not your typical desert heat".
"It's the desert, of course it's hot'- This is a DANGEROUS mind set!", the NWS in Las Vegas tweeted.
"This heatwave is NOT typical desert heat due to its long duration, extreme daytime temperatures, & warm nights. Everyone needs to take this heat seriously, including those who live in the desert."
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