U.S. President Barack Obama has cautioned Americans that the impact of Hurricane Irene “will be felt for some time.”

Mr. Obama said Sunday, “This is not over yet” and that the public is still at risk of flooding and power outages. He said it could take days or weeks for a recovery effort to wrap up on the U.S. east coast.

The president has declared an emergency for most of the states hit by Irene, including North Carolina, Virginia and New York, in order to make federal resources available to support response efforts.

Irene was downgraded from a hurricane to a tropical storm as it made its way up the East Coast. 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 80 kilometers an hour.

Forecasters say Irene will move into Canada by late Sunday.

The storm has killed at least 18 people and paralyzed ground and air traffic in its journey up the eastern U.S. coast. Major airports serving New York City are expected to stay closed until Monday. It was also unclear when New York's subway system would begin operating again.

More than 4 million homes and businesses have lost power.

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 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, D.C. area, which was hit with strong winds, heavy rain, localized flooding and falling trees.

The storm passed through some of the country's most densely populated areas. Suspected tornadoes spurned by the hurricane destroyed homes in Delaware and Virginia.