The United States sweltered in dangerously hot weather on Sunday, with major cities including New York, Philadelphia and Washington broiling in temperatures that rose 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 stifling heat that set records in a handful of states.


By Sunday afternoon, the National Weather Service (NWS) said the high temperatures would soon be over, but the severe weather would continue.


"The dangerous heat wave which enveloped much of the Midwest to the East Coast this weekend should finally break by Monday as a cold front drops southward," the agency said.


However it warned that "showers and thunderstorms… with heavy to excessive rainfall and severe weather possible" were expected across a swath of the east coast, and flash floods were a risk.


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


Earlier in the day the NWS said "numerous stations" stretching from Massachusetts in the north to North Carolina in the south "were reporting heat index values between 100-110" degrees, with the highest values found in the southeast of the state of Virginia.


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


"We're almost near the end of the heat emergency. Temperatures will start to go back down tonight," New York Mayor Bill de Blasio tweeted, asking the city's residents to "keep hydrated and keep cool."


The NWS said it had recorded all-time high temperatures on Saturday in seven places in the US including Manchester, New Hampshire, Atlantic City, New Jersey and New York's JFK airport.


US media has blamed the heat wave for at least six deaths, including a hiker who had been found unconscious on a trail outside Washington on Saturday and two people who died 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.