WASHINGTON (AFP) - Americans braced for a second - and equally scorching - day of dangerously hot weather Sunday, with daytime temperatures forecast to approach 38 deg C across a number of major US cities, including New York, Philadelphia and Washington.

An oppressive heat wave stretching from the Midwestern plains to the Atlantic coast had nearly 150 million people struggling to stay cool amid scorching temperatures on Saturday.

Heat index values - combining the effect of heat and humidity - could reach 43 deg C to 46 deg C, particularly in the east, the National Weather Service (NWS) said.

It warned that "dangerously high temperatures and humidity could quickly cause heat stress or heat stroke if precautions are not taken."

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

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

The heat wave already claimed at least three lives, including two earlier in the week in the eastern state of Maryland.

In Arkansas, former NFL player Mitch Petrus died on Thursday after working outside his family's shop. The 32-year-old died of heat stroke, the Pulaski County coroner was quoted as saying in US media.

Heat warnings have also been issued for parts of eastern Canada.

In New York, Mayor Bill de Blasio declared a heat emergency.

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

The estimated 4,000 participants, many of whom had travelled long distances to race, will receive full refunds of entry fees of up to US$399 (S$543), organisers said.

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

In Washington, a popular outdoor jazz concert at the National Galley of Art was cancelled.

'SERIOUS, SERIOUS STUFF'

New York City opened 500 cooling centres for residents.

"Saturday is going to be really, really bad, on through Sunday," de Blasio said in a warning to the city. "This is serious, serious stuff."

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."

The city's electrical grid was so far handling the extra demand, which came just a week after a major outage, blamed on mechanical problems, left tens of thousands of Manhattan residents in the dark.

In Washington, the morning low of 27 deg C was just one degree below the record set in 2015. There was a good chance to hit 37 deg C today for the first time since 2016, The Washington Post reported.

Triple-digit temperatures in the US Northeast are unusual.

The Philadelphia Inquirer suggested, tongue in cheek, that locals might want to seek relief in normally sweltering Miami or Phoenix, where it would be much cooler.

Philadelphia looked likely to set a new record for the hottest July 20 since 1930.

In Boston, where the weather service said that Saturday and Sunday would be "major scorchers," city officials scrapped entry fees at public pools.

In the far north-west, temperatures soared earlier this month in the state of Alaska, which straddles the Arctic Circle, with the largest city Anchorage hitting an all-time high of 32 deg C.

Earlier in the week, the National Weather Service office in the Midwestern city of Omaha baked a tray of biscuits - savoy breakfast bread similar to scones - on the dashboard of a parked car. After nearly eight hours and with temperatures on the pan reaching 85 deg C, 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.