More than three million people have been left without power amid a record heat wave in the eastern United States and deadly thunderstorms that downed power lines along mid-Atlantic region.

The violent storms have left at least 13 people dead, authorities said on Saturday.

Six people were reported killed in Virginia, including a 90-year-old woman asleep in bed when a tree slammed into her home.

Two young cousins in New Jersey were killed when a tree fell on their tent while camping. Two were killed in Maryland, one in Ohio and one in Washington, DC.

Statewide emergencies were declared in District of Columbia, Ohio, Virginia and West Virginia because of damage from overnight storms, which brought hurricane-force winds along a 600km stretch.

Forecasters predicted more severe thunderstorms coupled with renewed heat on Saturday.

Restoring power in some areas could take up to a week with utilities in Ohio and Virginia described damage as catastrophic.

"It's going to be a while before some folks get power, and with the heat, that's our big concern," said Bob Spieldenner, a spokesman for the Virginia Department of Emergency Management.

More than one million Virginia residents had no power in the worst outage not linked to a hurricane in the state's history, he said.

The storm also knocked out passenger rail services between Washington and Philadelphia, Amtrak railways said in a statement.

Power companies called in crews from utilities in neighbouring states to help restore electricity.

Record heat

The widespread power outages came as the National Weather Service forecast more heat and severe thunderstorms across the Ohio Valley and into the northern mid-Atlantic states.

State and local officials urged residents to seek air-conditioned areas, drink lots of water and wear light-colored clothing. They also called for people to watch out for those most vulnerable to high heat - the elderly, small children and the mentally ill.

"Our biggest concern right now is temperatures going up to 100 degrees today," said Ed McDonough, spokesman for the Maryland Department of Emergency Management.

People chose to escape the heat in shopping malls or hotels. Drivers navigated through intersections left unguided by dead traffic lights.

In suburban Washington, emergency call centres were without power and residents were told to go to police or fire stations if they needed help.

Others were asked to conserve water. Cell phone coverage was spotty.

Records for June were broken on Friday in Washington, Atlanta, Nashville, Tennessee and Louisville, Kentucky. The temperature hit at least  40 Celsius in all four cities, according to the National Weather Service.

The high heat prompted the AT&T National golf tournament at the Congressional Country Club in Bethesda, Maryland, to close the competition to spectators and volunteers on Saturday.