Financial markets reopened with New York Mayor Michael Bloomberg ringing the opening bell at the New York Stock Exchange, and packed buses took residents back to work with the subway system halted after seawater flooded its tunnels.

John F. Kennedy and Newark airports reopened with limited service after thousands of flights were cancelled, leaving travellers stuck for days. New York's LaGuardia Airport, the third of the airports that serve the nation's busiest airspace, was flooded and remained closed.

It will take days or weeks to recover from the massive power and mass transit outages.

With six days to go before the November 6 elections, President Barack Obama will visit storm-ravaged areas of the New Jersey shore, where Sandy made landfall on Monday.

He will be accompanied by Republican Governor Chris Christie, a vocal backer of presidential challenger Mitt Romney. Nevertheless, Christie has praised Obama and the federal response to the storm.

The storm killed 27 people in New York state, including 22 in New York City, and six in New Jersey. Seven other states reported fatalities. One disaster-modelling company said Sandy may have caused up to $15 billion in insured losses.

Sandy killed 69 people in the Caribbean last week before it slammed into the U.S. East Coast and pushed inland, dumping snow in the Appalachian Mountains and other inland areas.

Remnants of the storm churned slowly over Pennsylvania on Wednesday, the National Weather Service said. Winter storm warnings were in effect from southwestern Pennsylvania to eastern Tennessee.

Battered by a record storm surge of nearly 14 feet (4.2 meters) of water, large sections of New York City remained submerged under several feet of water. In the city's borough of Staten Island, police used helicopters to pluck stranded residents from rooftops.

Across the Hudson River in Hoboken, New Jersey, members of the National Guard arrived to help residents pump floodwater from their homes, the city said on Twitter.

More than 8.2 million homes and businesses remained without electricity across several states after trees toppled by fierce winds tore down power lines.

In New Jersey, Christie said it could take seven to 10 days before power was restored statewide.

Subway and commuter tunnels under New York City, which carry several million riders a day, were under several feet of water.

In the lower half of Manhattan, a quarter of a million residents remained without power after a transformer explosion at a Con Edison substation Monday night.

CRIPPLED COMMUTE

New York City likely will struggle without subway service for days, authorities said. Buses were operating on a limited basis and many residents were walking long distances or scrambling to grab scarce taxi cabs on the streets.

Sunday's New York Marathon will go on as scheduled, but Wednesday night's Halloween parade through Greenwich Village was postponed. On Broadway, the Theater League announced that most shows would resume performances on Wednesday. Shows had been cancelled since Sunday due to the storm.

In New Jersey, Christie took a helicopter tour of the devastation on Tuesday along the shore, where boats were adrift, boardwalks washed away and roads blocked by massive sand drifts. He stopped in the badly damaged resort towns of Belmar and Avalon.

"I was just here walking this place this summer, and the fact that most of it is gone is just incredible," he said at one stop.

Sandy hit the East Coast with a week to go to the November 6 presidential election, dampening an unprecedented drive to encourage early voting and raising questions whether some polling stations will be ready to open on Election Day.

Obama faces political danger if the government fails to respond well, as was the case with his predecessor George W. Bush's botched handling of Hurricane Katrina in 2005.

Obama and Romney put campaigning on hold for a second day on Tuesday but Romney planned to hold rallies in the battleground state of Florida on Wednesday and Obama seemed likely to resume campaigning on Thursday.

Sandy became the biggest storm to hit the United States in generations when it crashed ashore with hurricane-force winds on Monday near the New Jersey gambling resort of Atlantic City.

(Additional reporting by Michael Erman, Anna Louie Sussman, Atossa Abrahamian, Michelle Nichols, Ed Krudy, Chris Michaud and Scott DiSavino in New York and Ian Simpson in West Virginia; Writing by Daniel Trotta and Ellen Wulfhorst; Editing by Eric Beech)

• A Permanent Economic Emergency