Millions of people are at risk of life-threatening flooding when Hurricane Sandy plows into the northeastern US, the director of the National Hurricane Center said Sunday.

"The system is so large that I would say millions of people are at least in areas that have some chance of experiencing either flash flooding or river flooding," Rick Knabb told reporters during a telephone conference call.

He also said the storm could produce peak surges at high tide of six to 11 feet in the Long Island Sound, NY Harbor, Raritan Bay, a swath of the most heavily populated portions of the US coastline.

"Don't focus on the fact that this is a category one hurricane," he said, referring to the Sandy's current low rating on the five-rung Saffir-Simpson scale.

"The storm surge threat for a large area is a life-threatening hazard," he emphasized.

NHC forecaster Todd Kimberlain said the storm has the potential to break a number of different records.

"I think it's fair to say it would be up there. If it's not historic it would be near historic," he told AFP.

"The storm surge in Irene was four or five feet (1.2 to 1.5 meters)," Kimberlain said, referring to a 2011 hurricane that killed 47 people and caused an estimated $10 billion of damages in the US, well below projections for Sandy.

He added, "The surge is historically the most life-threatening, but we don't want to minimize the effect that the winds could have."

"We think there is a strong likelihood of hurricane force winds over a large portion of the mid-Atlantic and southern New England coast."

He drew comparisons to the so-called "Perfect Storm," which killed 13 people and caused $200 million in damages when it hit the northeastern US in October 1991.

"But even there, the strongest winds were confined to southern New England.

"This is going to be a more expansive region, and for that reason, I think the potential is there to be more damaging."

Kimberlain said Sandy's impact could reach well into the billions of dollars.