AT least 13 people have died and three million were left without power after violent storms pummelled the eastern United States.

At least six were killed in Virginia, including a 90-year-old woman when a tree fell on her home.

Two young cousins in New Jersey were killed when a tree fell on their tent while camping. Two people were killed in Maryland, while single deaths were reported in Ohio, Kentucky and Washington.

Energy officials said the power cuts would not be repaired for several days, likening the damage to the effects of a hurricane.

Emergencies were declared in Maryland, West Virginia, Ohio, the District of Columbia and Virginia, where governor Bob McDonnell said the state had its largest non-hurricane outage in history. In West Virginia, 232 passengers spent Friday night on a train that was blocked on both sides by trees.

In Illinois, storm damage forced the transfer of dozens of maximum-security, mentally ill prisoners from one prison to another.

In some suburbs of Washington, emergency call centres were out of service, with residents told to call local police and fire departments. Huge trees fell across streets, crushing cars underneath, while others fell onto the fairway at the AT&T National golf tournament in Maryland.

Mobile phone and internet service was erratic, petrol stations shut down and residents were urged to conserve water until power returned to sewage plants.

Power cuts left the region without air conditioning as temperatures soared in Baltimore and Washington, reaching 40C on Friday.

Damage stretched from Indiana to New Jersey.