The storm also caused widespread transport havoc and kept the federal government closed for a second day.

Downgraded to a tropical storm late Thursday, Isabel toppled thousands of trees and power lines in North Carolina, Virginia, Maryland and Washington after landfall Thursday, packing 160-kilometer-an-hour (100-mile-an-hour) winds at its peak.

The storm moved north into Canada Friday as a massive clean-up operation got underway, but dozens of flights remained cancelled after an estimated 1,500 were called off the day Isabel hit.

Most of the 21 deaths were caused by traffic accidents as trees and electricity poles were felled by the storm.

Virginia was the worst-hit state, with fourteen fatalities reported by the state's Department of Emergency Management.

One power worker in North Carolina was electrocuted as he tried to reconnect supplies at a power substation in driving rain, a woman was crushed to death in her car by a tree and two people died in Maryland as a result of the storm.

A man in Maryland was killed when his car was struck by a falling power pole.

In Pennsylvania, a man was killed when his car was blown off the road in the strong winds and a man drowned off the coast of Rhode Island after being swept from the shore by high waves.

More than 2.7 million people remained without power Friday including 1.5 million in Virigina, over 450,000 in North Carolina, 200,000 in Pennsylvania and thousands across in Maryland, greater Washington and West Virginia.

Virginia's Dominion Power said it could take up to a week to get all its customers reconnected.

"This outage is the worst in Virginia's history." said Jimmy Staton, a Dominion senior vice president.

Utilities warned people not to touch downed power lines, especially in flooded areas.

Companies handed out dry ice to the public to help preserve food. In Virginia, some district authorities advised the public to boil water before drinking because of possible contamination.

Virginia Governor Mark Warner paid tribute to emergency workers who fought through the night to rescue people trapped in floods and to free debris that was causing floods. "There were a lot of heroic stories," he said.

Along the Outer Banks, a chain of islands off the North Carolina coast, dozens of houses were flooded and roofs ripped off, and roads were cut off.

President George W. Bush declared Virginia, Maryland and North Carolina emergency disaster areas so they could claim federal aid. Virginia reported that at least 15,000 people had sought refuge in 66 shelters around the state, due to heavy flooding.

The Potomac River flooded parts of Alexandria, just south of Washington, and streets around Baltimore's harbour were under one meter (3 feet) of water.

The cost of the massive clean-up operation is expected to run to several billion dollars, according to Philip Crowley, a spokesman for the Insurance Information Insitute in New York.

"We could see the economic impact of hurricane Isabel running into several billions of dollars," Crowley, said explaining the hurricane had a broader economic impact due to the closure of the federal government.

Federal government offices in Washington closed Thursday as Isabel bore down on the US capital, and 360,000 federal workers were given Friday off as well.

The city's metro transit system was reopened, after shutting down Thursday before the storm hit.

The storm shut down Washington's Reagan National Airport and other smaller airports throughout the region.

Isabel was still blowing at about 65 kilometers (40 miles) an hour Friday as it moved into Canada.

TERRA.WIRE