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

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.

"This extreme heat is dangerous and can be life-threatening," NYC Emergency Management said on X, adding that people should "stay indoors in air-conditioned spaces as much as possible."

During rush hour on Tuesday morning, a power outage halted several subways lines in New York City, with authorities saying they were checking whether it was due to the heat.

"It was breathable, but everybody's so compact," subway rider Natalie Henry said.

Swimming pool hours have been extended at New York state facilities, and cooling centres have been set up to help people shelter from the heat.

According to preliminary data from NYC Emergency Management, the city's LaGuardia Airport hit 100F (37.7C) on Tuesday afternoon, which would be a record high for this day in July.

But that would still be way short of the all-time high of 107°F recorded at LaGuardia Airport on 3 July 1966, the NWS in New York told the BBC.

On Sunday, Tampa, down in the Sunshine State of Florida, hit 100F, breaking the previous record of 99F in 2020. (BBC)