The United States sweltered in dangerously hot weather on Sunday, with major cities including New York, Philadelphia and Washington broiling in temperatures rising into triple digits.

An oppressive heat wave stretching from the Midwestern plains to the Atlantic coast had nearly 150 million people struggling to stay cool in the stifling heat.

‘We're almost near the end of the heat emergency. Temperatures will start to go back down tonight. But today's heat index could still be as high as 110 degrees (43 degrees Celsius),’ New York Mayor Bill de Blasio tweeted.

‘Please continue to take precautions. Keep hydrated and keep cool.’

The heat was expected to continue through late Sunday however, as a high-pressure system off the Atlantic coast ushered in steamy, subtropical air.

Around 95 million people were under a heat warning or advisory for Sunday, down from Saturday's 157 million.

The National Weather Service (NWS) predicted ‘more very hot and humid conditions’ for the area from Washington to Baltimore, with highs ‘close to 100 degrees.’

A cold front stretching between the Central Plains and the Great Lakes region could bring cooler weather and thunderstorms by the beginning of the week, the NWS added, bringing a risk of flash floods to some areas.

But for Sunday, people were urged to stay hydrated, watch out for the sick and elderly, stay inside as much as possible and not leave children or animals in cars.

The heat wave has already been blamed for at least six deaths, including two earlier in the week in the eastern state of Maryland.

In Arkansas, 32-year-old former NFL player Mitch Petrus died of heatstroke Thursday after working outside his family's shop.

The New York City Triathlon, which had been scheduled for Sunday, was canceled for the first time since its founding in 2001.

Life Time, which produces the race, donated more than 12 tons of water and Gatorade Endurance drink meant for competitors to be distributed to New Yorkers in need, CBS reported.

Meanwhile, the two-day OZY Fest -- a food, comedy and music festival set for Central Park -- was also called off.

In Washington, a popular weekly outdoor summer jazz concert at the National Gallery of Art Sculpture Garden was canceled.

- 'Deadly conditions' -

New York City opened 500 cooling centers for residents.

At least three public defenders said on Twitter that inmates in New York's notorious Rikers Island jail complex were suffering with no air conditioning, and that some guards had turned off fans as punishment, resulting in ‘deadly conditions.’

The Brooklyn Defender Services legal aid group said some inmates didn't have summer clothing, only long underwear provided by the group last winter.

Top officials from the city's Department of Corrections were at the facility monitoring the response to the heat wave to ‘protect health and safety of everyone in the facility,’ de Blasio wrote on Twitter.

The department said in a statement to AFP that extra staff were on hand to distribute summer clothing, and clinics were open around the clock to treat heat-related symptoms.

Those in units without air conditioning were given access to fans, ice, water and ‘multiple cool showers.’

Earlier in the week, the National Weather Service office in the Midwestern city of Omaha baked a tray of biscuits -- savory breakfast bread similar to scones -- on the dashboard of a parked car.

After nearly eight hours and with temperatures on the pan reaching 185 degrees, the pastries were almost fully cooked.

Climate data showed June was the hottest month on record worldwide, with a heat wave across Europe smashing national records.