Millions of people across the US have been warned to stay indoors as about half the country swelters under a heatwave, BBC reported.

Heat advisories are affecting 168 million Americans in the mid-west, south-east and parts of the north-east, according to the National Weather Service (NWS). 

New York Governor Kathy Hochul urged New Yorkers to check on vulnerable neighbours, saying that heat was the number one cause of extreme weather fatalities in the state.

The weather is forecast to cool on Thursday a day after storms roll through the region, according to BBC.