Tens of millions of Americans braced themselves up for a potentially devastating Hurricane Sandy as its wind speed crossed the dangerous 90 miles per hour, threatening "unprecedented damage" with forecasters warning that it will slam cities from New York to Washington.

Authorities declared a state of emergency from North Carolina to Connecticut as the superstorm made its way up the Atlantic on a collision course with two other weather systems, that could turn it into one of the most deadly storms to hit the US. It could affect as many as 60 million people.

President Barack Obama suspended his election campaign and rushed to the White House to personally review preparations as the Hurricane Sandy - being dubbed as a 'Frankenstorm' - threatened to hit America's East coast.

Obama held an emergency situation room meeting with his top advisors and officials on hurricane preparedness.

Meanwhile, the Nuclear Regulatory Commission (NRC) has deputed inspectors at all nuclear power plants that could potentially experience impacts from the storm.

At least eight nuclear power plants are falling on the path of Hurricane Sandy including Indian Point, in New York.

"The inspectors are independently verifying that plant operators are making the proper preparations, are following relevant procedures and are taking appropriate actions to ensure plant safety before, during and after the storm," it said in a statement.

Sandy is threatening to become the largest storm to hit the mainland in US history. It was producing sustained winds of 90 miles per hour by 11 am Monday, up from 75 miles on Sunday night.

An expected storm surge at midnight could raise water levels to 11 feet above normal high tide, bringing "the potential to cause unprecedented damage."

Obama warned that millions of people would be affected, as "a serious and big" storm Hurricane Sandy headed towards the East Coast. "This is going to be a big storm. It's going to be a difficult storm," Obama said.

"We anticipate that the center of the storm is going to hit landfall sometime this evening. But because of the nature of this storm, we are certain that this is going to be a slow-moving process through a wide swathe of the country, and millions of people are going to be affected," Obama told White House reporters, soon after holding situation room meetings.

"We're making sure that food and water, and emergency generation is available for those communities that are going to be hardest hit," Obama said as he asked his countrymen to strictly follow the instructions of authorities.

Obama cautioned that the transportation and the power to be effected in the storm hit areas might take a long time before the services are resumed.

"Transportation is going to be tied up for a long time. And probably the most significant impact for a lot of people, in addition to flooding, is going to be getting power back on," he said.

In its latest advisory on Monday morning, the National Hurricane Centre said Sandy is forecast to turn northwestward and "expected to bring life-threatening storm surge and coastal hurricane winds plus heavy Appalachian snows."

Nine US states, including Connecticut, New Jersey, Pennsylvania and Maryland declared states of emergency.

"This is the most catastrophic event that we have faced and been able to plan for in any of our lifetimes," Connecticut Governor Dan Malloy warned.

The Federal Emergency Management Agency said cost from wind damages alone could reach USD three billion in the wake of Sandy with losses totalling several billions of dollars more due to damage caused by water, toppled trees and power outages.

Public transport system, the lifeline of millions of commuters, was mandatorily closed in cities like New York, Boston, Washington and New Jersey. Nearly 7,000 flights were cancelled as airlines scrambled to get their planes out of the storm's path.