The US Northeast began crawling back to normal on Wednesday after monster storm Sandy crippled transportation, knocked out power for millions and killed at least 50 people in nine states with a massive storm surge and rain that caused epic flooding.

Financial markets reopened after taking two days off with New York Mayor Michael Bloomberg ringing the opening bell at the New York Stock Exchange, and packed buses took commuters 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 canceled, leaving travelers 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.

Homeland Security Secretary Janet Napolitano said the storm may be the most expensive in US history.

It will take days or weeks to recover from the massive power and mass transit outages. Nearly 6 million customers are without electricity in the region.

The storm is causing more havoc as it moves north toward Canada. Flooding warnings are in effect in the Great Lakes region and heavy snow has been falling in the Appalachian mountains.

With six days to go before the November 6 elections, President Barack Obama will visit storm-ravaged areas of the New Jersey shore, where the Sandy crashed ashore on Monday as the largest storm to hit the United States in generations.

Obama 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 each in New Jersey and Pennsylvania. Six other states reported fatalities. One disaster-modeling company said Sandy may have caused up to $15 billion in insured losses.

Sandy killed 69 people in the Caribbean last week as a hurricane before it slammed into the U.S. East Coast with winds of about 130 km per hour and pushed inland.

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.

"Now we are looking at flooding on Lake Erie, possibly Lake Michigan," Napolitano said. "We're looking at secondary flooding downstream as rivers fill with the remnants of Sandy and the water has to go somewhere.

"We are now in recovery mode - response and recovery - we are moving large amounts of resources into the affected areas. It will be one of the most, probably if not the most extensive and expensive ... (storms) in our nation's history," she said.

Battered by a record storm surge of nearly 14 feet (4.2 meters), 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 helped residents pump floodwater from their homes, the city said on Twitter.

Chest-high floodwaters rushed into the streets in a flash on Monday night just after the power went out, and by Wednesday morning the water was still knee high in many areas of Hoboken.

National Guard vehicles patrolled the streets but emergency vehicles were scarce, witnesses said.

Some 5.9 million homes and businesses in several states remained without power on Wednesday morning, down from a high of nearly 8.5 million, which surpassed the record 8.4 million customers who went dark from last year's Hurricane Irene.

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

In the southern half of Manhattan, a quarter of a million residents remained without power after a transformer explosion at a Consolidated Edison substation Monday night and the flooding of another. Citywide, some 760,000 customers lacked power on Wednesday.

Sunday's New York Marathon will go on as scheduled, but Wednesday night's Halloween parade through Greenwich Village has been postponed. On Broadway, shows that had been canceled since Sunday were due to mostly resume on Wednesday, the Theater League announced.

Sandy hit the East Coast with a week to go before the Nov. 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 will resume campaigning on Thursday with a stop in Nevada.

Obama continued to receive updates overnight from his team on recovery efforts and he visited the federal government's storm-response center in Washington before he was due to fly off and meet Christie.-Reuters