A heat wave baking the eastern United States in record temperatures is set to continue after deadly storms killed at least 12 people, downed power lines from Indiana to Maryland and left more than three million people without power.

Emergencies were declared in Washington DC, Ohio, Virginia and West Virginia yesterday because of damage from overnight storms.

Hurricane-force winds were felt across a 800km (500 mile) stretch of the mid-Atlantic region.

President Barack Obama authorized the Federal Emergency Management Agency (FEMA) to coordinate all disaster relief efforts in storm-ravaged Ohio.

The storms' rampage was followed by roasting temperatures that topped 38C (100F) in several southern cities, including Atlanta, where the mercury hit 41C (106F), according to Accuweather.com.

Heat advisories remained in effect across the southeast and lower half of the Mississippi valley, with "triple-digit temperatures expected across the southern third of the country," the National Weather Service said.

Power crews worked into the night to try to restore service to homes and businesses, and officials said in some areas the job could take up to a week.

Utilities in Ohio, Virginia and Maryland described damage to their power grids as catastrophic.

Six people were killed in Virginia in storm-related incidents, and more than one million customers were left without power in the worst outage not linked to a hurricane in the state's history, a spokesman for the Virginia Department of Emergency Management said.

Two Maryland residents died in the storm - one struck by a falling tree in Anne Arundel County, the other electrocuted after a tree crashed into a house in Montgomery County.

In New Jersey, two cousins aged two and seven were killed by a falling tree in a state park.

Heat was blamed for the deaths of two brothers, aged three and five, in Bradley County, eastern Tennessee. They had been playing outside in 41C heat