Washington, Aug 29 (IANS): As Hurricane Irene petered out leaving 18 dead across seven states and impacting millions with strong winds and drenching rains, authorities prepared to fight dangerous, damaging flood waters along the East Coast.

The cost from wind damage alone is expected to top $1 billion, with downed power lines leaving more than four million people without electricity, according to US government estimates.

"I want people to understand that this is not over," President Barack Obama said Sunday evening from Washington. "The impacts of this storm will be felt for some time, and the recovery effort will last for weeks or longer."

Flood warnings and watches were in effect Sunday night for much of New Jersey, Pennsylvania, eastern New York, Connecticut, Massachusetts, Vermont, New Hampshire and Maine.

Also hard-hit was New Jersey, where initial fears about coastal flooding -- which had prompted the evacuation of more than 1 million people from the shore -- had given way to fresh concerns about inland flooding.

In New York City, officials worked Sunday night to return the city to normal. Hours earlier, the Hudson River overflowed in lower Manhattan, receding only after massive amounts of water spilled over jogging paths and into at least one nearby apartment building.

Area's three major airports -- Newark Liberty in northern New Jersey and LaGuardia and John F. Kennedy in the New York City boroughs of Queens -- will reopen Monday, two days after they shut down.

New Jersey Transit rail service is suspended "until further notice," except for the Atlantic City rail line, but light rail and bus service will resume Monday.

By 8 p.m., Tropical Storm Irene had maximum sustained winds of 50 mph and was nearing the US-Canadian border, according to the National Hurricane Centre.