Millions in the United States remain without power or transportation in the wake of Sandy, which caused dozens of deaths and billions of dollars of damage along the country's East Coast.
The worst damage was in the New York area, the most densely populated stretch of land in the country.
At least 18 people were killed in the city, according to Mayor Michael Bloomberg.
The storm also caused widespread damage in neighbouring New Jersey, where homes and businesses along the coast sustained extensive damage.
More than two million households in New Jersey lost power - twice the impact of Hurricane Irene, the storm which battered the East Coast last year.
US declares 'major disaster' in New York
Nationwide, some eight million homes were without electricity, and more than one million people were still under evacuation orders.
More than 40 storm-related deaths were reported across the states of Massachusetts, Maryland, Connecticut, New Jersey, Pennsylvania and West Virginia. Sandy had already killed more than 60 people in the Caribbean in the past week.
Federal government offices in Washington, which was spared the worst of the storm, were closed for a second day on Tuesday, and schools up and down the coast remained closed.
Parts of New York, New Jersey and Connecticut have been declared "disaster zones", meaning more relief funds will be available to help those states recover.
In West Virginia, up to 60cm of snow fell in mountainous areas, and a blizzard warning for more than a dozen counties remains in effect until Wednesday afternoon.
Brandon Williams, a resident of Elkins, said the situation there was "crazy".
"You can't go nowhere," he told Al Jazeera. "Trees in the road. The interstate's shut down. Not much you can do ... They got about four tractor-trailers jack knifed on that mountain and two of them are side by side."
The National Hurricane Centre said Sandy came ashore as a "post-tropical cyclone", meaning it still packed hurricane-force winds but lost the characteristics of a tropical storm.
It had sustained winds of 129km per hour (kph), well above the threshold for hurricane intensity; it brought record flooding to coastal areas, and several feet of snow in mountainous areas such as the state of West Virginia.
Bloomberg said on Tuesday that the upcoming challenges facing the city in the coming days "are enormous".
"Make no mistake about it: This was a devastating storm, maybe the worst we have ever experienced," he said.
"You should expect, given the extent of damage, power will be out for two or three days, maybe even longer than that."
The storm brought a surge of almost 14ft to the business district of Manhattan, well above the previous record of 10ft during Hurricane Donna in 1960, according to the National Weather Service.
Public transport in the city will remain closed until further notice, and schools and airports will also remain closed on Tuesday.
It could take days to pump the water out of New York's flooded subway tunnels, and the runways at the city's two major airports are both underwater as well.
New York University's Tisch hospital was forced to evacuate more than 200 patients, among them babies on respirators in the neonatal intensive care unit, when the backup generator failed.
The flooding also hampered efforts to fight a massive fire that engulfed more than 50 homes in Breezy Point, a private beach community on the Rockaway barrier island in the New York borough of Queens.
The New York Stock Exchange remained closed for the second day in a row on Tuesday, but officials said they planned to reopen the exchange on Wednesday.
Dozens of companies have delayed their earnings reports because of the storm, which may also disrupt the labour department's monthly jobs report, scheduled for release on Friday.
Some residents refused to leave Atlantic City, New Jersey, despite an order to evacuate
The storm has shifted attention away from the US presidential campaign, with just one week to go before election day on November 6.
Barack Obama, the US president, will inspect the damage in New Jersey on Wednesday along with the state's Republican governor, Chris Christie.
Christie has been a prominent surrogate for Mitt Romney, the Republican presidential nominee.
But he praised Obama on Tuesday, calling his response to the storm "outstanding," and dismissed questions about whether Romney would visit the state for a campaign appearance.
"If you think right now I give a damn about presidential politics, then you don't know me," he said in a television interview.
Romney appeared at an event in Ohio, a crucial battleground state, but avoided talking politics, instead urging people to donate food, bottled water and other supplies to storm victims.
"We have heavy hearts this morning with all the suffering going on in a major part of our country," he said.
Some US publications have highlighted remarks made by Romney in 2011 appearing to favour the elimination of federal agencies such as the disaster relief organisation FEMA.
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