About one in five Americans - around 70 million people - are living under heat alerts, with temperatures expected to soar.
People in the north-east and Midwest have been told to prepare for days of extreme heat, with temperatures of 105 degrees Farenheit or 41 degrees Celsius possible in some areas.
Elsewhere in the country, firefighters are battling a wildfire north of Los Angeles that has forced hundreds of people to leave their homes.
The National Weather Service has described this week's expected extreme temperatures as the region's first significant heatwave of the season.
In its latest update on social media site X, it said the Midwest, Great Lakes, Northeast and Mid-Atlantic regions could all be affected until next week.
Scientists say extreme weather events are becoming more frequent and intense as a result of climate change.
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