Storm-hit US states declare emergencies

States of emergency have now been declared in four states and the US capital after violent storms cut a swathe through the east of the country.

Thirteen deaths have been linked to the sudden storms, which, packing hurricane-strength winds, uprooted trees and downed power lines.

Amid an intense heatwave, three million people were left without power.

Power companies are warning that some may not have electricity restored for up to a week.

Officials have warned that the heatwave - compounded by the loss of air conditioning due to power outages - could threaten the very young, old and sick.

In Bradley county, eastern Tennessee, the high temperature has been blamed for the deaths of two brothers, age three and five, who were playing outside in 105F (40.6C) heat, Reuters news agency reports.

Maryland, West Virginia, Virginia, Ohio and the District of Columbia have all now declared states of emergency.

Virginia Gov Bob McDonnell said the state - where six people died from the storms - had had its largest non-hurricane power outage in history.

"This is a very dangerous situation," he said, according to Associated Press.

Maryland Gov Martin O'Malley said the storms had been even more challenging than a hurricane, because unlike hurricanes they began without warning.

In Illinois, officials transferred 78 maximum-security inmates from a prison in Dixon after it suffered storm-related damage. Prisoners remaining in the Dixon jail were locked down - confined to their cells - while generators provided power.

According to a local newspaper , the transfer has prompted union officials to question plans to close correctional facilities amid already overcrowded conditions.

The storm damage also caused online disruption, with Netflix, Instagram and Pinterest services temporarily disabled.

Meanwhile, utility companies said they were working around the clock to repair damage which some described as catastrophic.

"We do understand the hardship that this brings, especially with the heat as intense at is. We will be working around the clock until we get the last customer on," said Myra Oppel, a spokeswoman for the utility Pepco, which serves Washington and its suburbs.

The National Weather Service has warned of more possible thunderstorms , saying it has three areas of particular concern: the northern Rockies, the mid-Mississippi River to Ohio Valleys and portions of the Mid-Atlantic into the Southeast, particularly the eastern Carolinas.

"The primary threats will be large hail and damaging winds; however, an isolated tornado can not be ruled out across southern Minnesota and Wisconsin as well as northern Iowa and Illinois," it said.