WASHINGTON — Utility crews made slow progress Sunday 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 (37.7 Celsius) range in many areas, 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," added 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.

Pepco, the utility that serves Washington and some of its suburbs, said it could take a week before all of its customers get their electricity back, reported WTOP all-news radio in the US capital.

Local authorities kept "cooling centers" open 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.

At least 11 people died in Friday's storms, according to US news media, and four states -- Maryland, Virginia, West Virginia and Ohio -- declared states of emergency. Some 3.7 million homes lost power.

"The power is slowly coming back on line," West Virginia Governor Earl Ray Tomlin told CNN on Sunday, adding however that "we still have over half a million people without power."

"We just once again ask people to be patient," he said. "It's going to take a few days."

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 first-hand and to praise "the courage and determination and professionalism" of those fighting the flames.