"It's pretty hot right now from the Plains states all the way through the Mississippi Valley to the East Coast," said Alex Lamers, a meteorologist with the National Weather Service's Weather Prediction Center.

Heat is the deadliest of all weather phenomena, according to the weather service, and cities from Chicago to New York urged residents to seek refuge at cooling centres set up in locations ranging from libraries to shopping malls.

Tourists visiting the monuments and museums on the National Mall in Washington endured temperatures approaching 37.8C.

Nighttime temperatures have remained so high in the heat wave, which started midweek in the Midwest, that Rockford, Illinois, for the first time since 1918 matched its all-time record by failing to fall below 27C on Friday.

High temperatures combined with high humidity make it feel even hotter, called the heat index.

"Heat stress tends to have a cumulative effect so if you're not recovering at night, especially if you don't have an air conditioning unit, you'll accumulate more heat stress," Lamers said.

"When it stays warm overnight, it provides a higher launch for temperatures the next day," he added.

Some outdoor activities, including the New York City Triathlon and a running race outside Washington, were cancelled. Others bent their rules to keep attendees cool and safe, including Yankee Stadium, which allowed fans to bring their own bottled water and shading umbrellas and offered hydration stations on all levels of the ballpark.

Sizzling temperatures were expected to persist through Sunday.

Australian Associated Press