Americans awoke to scenes of devastation Tuesday after megastorm Sandy battered the East Coast, flooding parts of New York City, paralyzing transport and leaving millions without electricity.

The storm weakened as it moved further inland but forecasters still warned of gale-force winds and flooding along the densely populated coast, where a massive fire broke out in New York City and a levee broke in New Jersey.

At least 13 people were killed in the United States and Canada as the storm roared ashore late Monday, pounding several major cities with heavy rain and hurricane-force winds that toppled trees and ripped down power lines.

President Obama declared that a "major disaster" had hit the states of New York and New Jersey, an order that cleared the way for federal grants and loans to help storm victims acquire temporary housing and repair damage.

Seawater coursed between the iconic skyscrapers of New York's financial district in lower Manhattan, flooding subways and road tunnels and shorting out the power grid, leaving a half-million households and businesses in the dark.

Further south, the sea surged over vast swathes of the eastern seaboard, turning coastal cities into ghost towns as the high winds grounded flights and shut down rail links, public transport and government offices.