U.S. weather authorities on Sunday issued a warning regarding the heatwave in 14 central and southeastern states, while millions of people continue to be without electric power in nine states after severe storms late last week.

The National Weather Service issued a warning concerning high temperatures in 14 states, particularly for vulnerable groups such as the sick and elderly, while millions of people continue to be affected by power blackouts and many are temporarily moving into air conditioned shelters to protect themselves from the heat.

On its Web page, the NWS said that some of the states affected by the heat would register temperatures of above 37 C (99 F) on Sunday and it warned the public to be alert for the symptoms of heatstroke, including cramps and fatigue.

The combination of high temperatures and an increase in the humidity index creates a situation where it is possible to develop illnesses related to the heat, the NWS said, recommending that people drink plenty of liquids, seek shelter in buildings with air conditioning and look out for vulnerable people and pets.

"We've had the largest blackout in history ... and we're asking people to have patience" because it will take several days to resolve the situation, West Virginia Gov. Earl Ray Tomblin told CNN on Sunday.

"We're asking them to stay at home, so that our teams can clean up the roads," Tomblin added, saying that half a million residents of his state continue to be without electricity.

A combination of storms and high temperatures on Friday night and Saturday morning resulted in the deaths of at least 12 people, six of them in Virginia, according to state authorities.

The greatest danger from the storms comes from falling trees and from hanging live wires.

In the U.S. capital, police said that two people were severely shocked when they touched a live wire. The man died and his wife is in critical condition at a local hospital.

The blackouts have also caused problems in public transport, delays at airports and economic losses for businesses without backup generators. Some churches cancelled their religious services on Sunday or transferred them to alternate sites with air conditioning.

In the state of Ohio alone, calculations are that around 900,000 people are without electric power, and Gov. John Kasich said that reestablishing service to all who have been affected could take up to a week.

The governments of Ohio, Maryland, Virginia and West Virginia on Saturday declared states of emergency due to the thunderstorms that pounded at least nine states on Friday night and Saturday morning.

That move allows them to receive aid from the Federal Emergency Management Agency, or FEMA, for cleanup operations and the reestablishment of electric power, among other tasks.

New Jersey Gov. Chris Christie said on Saturday that people in his state particularly need fuel, generators and communications equipment as a result of the storms. EFE