Tens of millions of US east coast residents are prepare for Hurricane Sandy, which could make landfall as the largest storm to hit the United States, bringing battering winds, flooding and even heavy snow.

The massive storm has already killed 66 people in the Caribbean.

It is headed toward a densely populated region that includes Washington, New York and Boston, and its effects could be felt for hundreds of kilometres, officials have warned.

It could be the largest storm to hit the United States, according to the National Oceanic and Atmospheric Administration.

New York and other big cities closed their transit systems and schools and ordered residents of low-lying areas to evacuate before a storm surge that could reach as 3.4m.

They warned that power outages could last for days.

The US government said it had granted administrative leave to non-emergency federal workers in the Washington DC area.

President Barack Obama asked residents to heed the orders of state and local authorities to protect themselves from Sandy.

"This is a serious and big storm," Mr Obama said after a briefing at the federal government's storm response centre in Washington.

"We don't yet know where it's going to hit, where we're going to see the biggest impacts."

Officials ordered many school closures, New Jersey casinos and Broadway theatres prepared to close, airlines got ready to halt flight activity in the New York area, and residents cleared store shelves of vital supplies and food.

New York Mayor Michael Bloomberg ordered the evacuation of low-lying areas of New York City, from upscale parts of lower Manhattan to waterfront housing projects in the outer boroughs, that are home to some 375,000 people.

"If you refuse to evacuate you're not only putting yourself at risk but also the first responders who will have put themselves at risk in an emergency," Mr Bloomberg told a news conference.

In Bridgeport, Connecticut, Fire Department Captain Frank Rivera was ordering residents of 300 waterfront homes to evacuate.

"A lot of them are on their way out but some of them are thinking about [Hurricane] Irene and how they didn't get damage and I tell them it's two different animals," Mr Rivera said, referring to the August 2011 hurricane that brought severe flooding to parts of New Jersey and Vermont.

Officials warned that flooding could be particularly severe since the storm's arrival coincides with a full moon, which normally means higher-than-usual high tides.

"The most worrisome aspect of Sandy is the high tides," said Rhode Island Governor Lincoln Chafee.

"With the full moon occurring Monday, with Sandy coming up the Bay, we're very concerned about flooding in our low-lying coastal areas.

New York City, New Jersey and Philadelphia transit authorities said they would begin shutting down service this afternoon. Maryland's transit system, serving some suburbs of Washington, said it would not open tomorrow.

Amtrak, the passenger rail service, cancelled nearly all service on the Eastern seaboard tomorrow and would halt its service north of New York along the Northeast corridor.

Transit systems in Washington and Boston said they planned to operate as usual tomorrow as long as it was safe to do so.

Airlines flying into and out of New York's three major airports were all expected to cease flight activity tonight, according to the Port Authority of New York and New Jersey.

Forecasters said Sandy was a rare, hybrid "super storm" created by an Arctic jet stream wrapping itself around a tropical storm, possibly causing up to 30cm of rain in some areas, as well as up to 90cm of snowfall in the Appalachian Mountains from West Virginia to Kentucky.

Nasdaq planned to open tomorrow despite the transit shutdown and evacuation orders, with big banks putting up key personnel in hotels overnight so that they would be able to make it in to work in the morning.

The Chicago Mercantile Exchange said it would suspend floor trading on the NYMEX oil market tomorrow, as it is located in the New York City evacuation zone near the Hudson River. It said electronic trading would go on as usual.

Worried residents in the hurricane's path packed stores, searching for generators, flashlights, batteries, food and other supplies in anticipation of power outages. 

Broadway's theatres planned to close tonight and remain closed tomorrow.

New York City, Boston and Washington were among the jurisdictions that announced schools would be shut tomorrow.

Sandy blew the presidential race off course, forcing Obama and Republican challenger Mitt Romney to cancel scheduled campaign stops.

It fuelled fears that the storm could disrupt early voting before the 6 November election.

Nearly 284,000 residential properties valued at $88bn are at risk for damage from storm-surge flooding, risk analysts CoreLogic said.

Some flights to and from US cancelled

The Department of Foreign Affairs has advised Irish citizens "not to travel to the US mid-Atlantic coastal region until Hurricane Sandy has passed and the extent of damage to travel and other infrastructure has been assessed by the US authorities".

The Dublin Airport Authority has said a number of transatlantic flights have been cancelled on Monday and Tuesday.

Passengers with plans to travel to the US in the next two days are advised to contact their airline before travelling to Dublin Airport.

Aer Lingus has cancelled its flights to and from New York  tomorrow due to the severe weather anticipated.

The following flights have been cancelled:

EI-105 Dublin to New York and EI-104 New York to Dublin

EI-111 Shannon to New York and EI-110 New York to Shannon

EI-109 Dublin to New York and EI-108 New York to Dublin

The airline has apologised for the disruption and has asked those scheduled to travel to visit its website.

British Airways has cancelled all flights to New York today and will suspend most services to and from the US east coast tomorrow.