Utility crews made slow progress Monday in restoring electricity to millions of households amid a record-setting heatwave in the eastern United States that showed no sign of abating soon.

Temperatures shot back into the 100-degree Fahrenheit (38 Celsius) range in many areas Sunday, prompting the National Weather Service to warn of the prospect of severe thunderstorms including large hail and damaging winds.

"Cities from St. Louis, Missouri, to Washington DC are forecast to approach or break daily record high temperatures for yet another day and there may be more all-time records broken," said AccuWeather, a private weather service.

Several million households made do without electricity two days after powerful thunderstorms knocked out power lines in nine states, notably in the mid-Atlantic region.

At least 14 people died in Friday's storms, according to US news media, in Virginia, Maryland, New Jersey, Ohio, Kentucky and the District of Columbia.

Pepco, the utility that serves Washington and some of its suburbs, said it may be a full week before service is restored to all its customers by teams of linemen that included reinforcements from as far as Oklahoma and Florida.

"The devastation is extensive and while we expect to have the vast majority of customers restored by the end of day Friday, restoration for some customers may extend into the weekend," said Pepco regional president Thomas Graham.

The situation got so bad that neighboring Canada has offered its helping hand, saying it was sending about 200 power restoration specialists to the Washington metropolitan area.

Canadian public utility Hydro One said the Canadians will start in the Washington, Baltimore and Virginia areas as early as Tuesday.

"Hydro One crews have a long standing history of assisting neighbouring utilities when help is needed the most, and this time is no exception," company vice president Len McMillan said in a statement.

Meanwhile, authorities in Washington put out a hyperthermia alert, saying the heat index -- which is the thermometer reading adjusted to take humidity into account -- in the afternoon was 101 F degrees.

"Cooling centers" remained opened in many urban areas as refuges for those -- notably the elderly -- unable to cope without air conditioning, and at least one major supermarket chain gave away free ice to all comers.

The District of Columbia's emergency management agency suggested going to a movie or a museum in order to beat the heat. "Bottom line is, stay (as) hydrated and cool as possible," it said on its Twitter feed.

In Baltimore, Maryland, the local Baltimore Gas and Electric utility said it had deployed 1,000 trucks to restore power to 306,000 customers in and around the major port city.

Storm debris was gone from major highways in the region, but downed branches and trees still littered secondary routes.

Responsible for the extreme early-summer weather has been a high pressure area parked over the southeastern United States -- the same slow-moving weather system blamed for a fatal wildfire in Colorado earlier in the week.

Firefighters supported by water bombers managed over the weekend to contain much of the Waldo Canyon inferno that killed three people, destroyed nearly 350 residences and left many hundreds homeless.

Colorado Governor John Hickenlooper, speaking Sunday on CNN, said it felt as if the worst was over. "But we also know that Mother Nature will be fickle out here," he added. "We're keeping ourselves very alert."

President Barack Obama visited Colorado on Friday to see the devastation firsthand and to praise "the courage and determination and professionalism" of those fighting the flames.