A crippling heat wave that has held large swathes of the United States hostage gave way slightly on Sunday -- but not before leaving dozens dead in several states, officials and local media said.

After days of sweltering highs around 100 degrees Fahrenheit (38 Celsius) in the central and eastern parts of the country, forecasters said that cooler air was slowly swooping south from Canada.

"Temperatures will drop, bringing some much needed relief," the National Weather Service said.

The relentless heat sparked health warnings and sent people to makeshift cooling shelters, shopping malls and overcrowded swimming pools in search of relief.

But the oven-like temperatures took their toll all the same, with the elderly and infirm especially vulnerable.

The eastern state of Maryland has recorded at least 13 heat-related deaths since June 8, with 11 of the victims aged over 65, Ed McDonough, a spokesman for the Maryland Emergency Management Agency, told AFP.

"We are hoping that this is it," he said in reference to the extreme weather.

In Ohio, three seniors with heart problems perished due to the heat after a fierce band of firestorms tore across several states on June 29, leaving them and millions of others without much-needed air conditioning after power cuts.

"I can confirm three heat-related deaths," Tamara McBride of Ohio's Emergency Management Agency told AFP. "However, there may be more."

In Chicago, where temperatures sank Sunday to more comfortable levels, the number of confirmed deaths linked to the record-setting weather soared to 18, the Chicago Tribune reported.

Officials in Illinois could not immediately confirm the figure.

In Indiana, meanwhile, an infant died after being left in a stiflingly hot car, according to the indystar.com website.

Deaths related to the stifling temperatures and heat-fueled storms were also reported in Virginia, New Jersey, Pennsylvania and Tennessee, local media said.

The blistering heat has also caused transportation-related woes.

Just outside Washington, the derailment Friday of a Metro subway train was linked to a "heat kink," the transit authority said. In the central state of Wisconsin, searing temperatures are being blamed for the buckling of highways.

However, with the somewhat refreshing relief comes the threat of severe thunderstorms and more power outages.

"Severe storms will bring damaging wind gusts and large hail from the Midwest to the Mid-Atlantic," The Weather Channel warned on its website.

The ominous headline comes as Pepco, one of the utility companies serving the Washington area, announced nine days after the devastating June 29 storm that it had restored power to "the final few customers" suffering from electricity cuts.