U.S. Homeland Security Secretary Janet Napolitano has cautioned people along the path of Tropical Storm Irene that even if the storm has passed through their area, danger remains.

Napolitano said Sunday that downed power lines, flooding, generator problems and fallen trees are all potential threats once the storm is gone.

Even as Irene continues on a path up the coastline of the northeastern United States, the Homeland Security chief said damage assessment has begun in the southern states of North Carolina and Virginia.

The National Hurricane Center says Irene has weakened since leaving the New York City area and is now traveling at about 40 kilometers an hour with maximum sustained winds of 95 kilometers an hour.

Forecasters say Irene will move into Canada by Sunday night. In areas where the storm already has passed, weather officials say the damage not as severe as they feared it would be.

The storm has killed at least 12 people and paralyzed ground and air traffic in its journey up the eastern U.S. coast.

New Jersey Governor Chris Christie has predicted record-level flooding for parts of his coastal state, and Pennsylvania Governor Tom Corbett has warned that rivers in the affected areas of his state may not crest until Tuesday or Wednesday.

Irene was downgraded to a tropical storm as it approached New York City early Sunday, but it still managed to flood some streets and a traffic tunnel in lower Manhattan, as the East and Hudson rivers rose with high tide.

But the effects of the storm did force two nuclear reactors to cease operations – one in Maryland and one in New Jersey. Authorities say the facilities are safe, and no one has been harmed.

Irene blasted ashore in North Carolina early Saturday, flooding streets and toppling trees with winds of 140 kilometer per hour. The storm later moved into the Washington, DC area, which was hit with strong winds, heavy rain, localized flooding and falling trees.

More than four million homes and businesses in the eastern U.S. lost power because of the storm, which is passing through some of the country's most densely populated areas. Suspected tornadoes spurned by the hurricane destroyed homes in Delaware and Virginia.

In New York, Mayor Michael Bloomberg ordered the city's first ever mandatory evacuation in low-lying areas of the city, including the Wall Street financial district. And in another first, New York's entire public transit system, including subway trains and buses, was shut down before the storm, as well as all three of New York's major airports.

Irene was the first hurricane to seriously threaten the United States in three years. Besides the U.S. fatalities, Irene killed at least one person in Puerto Rico and two in the Dominican Republic, and destroyed homes in the Bahamas.

It came almost six years to the day after Hurricane Katrina flooded the southern city of New Orleans, killing more than 1,800 people and forcing more than a million residents from their homes.