The Associated Press reported the U.S. deaths happened in New Jersey, New York, Maryland, Pennsylvania, West Virginia, and Connecticut.
The cause of death was not immediately clear in most cases, but Reuters reported that at least two of the deaths were in Queens, New York. One man died when a tree hit a house, and a woman died in the some borough after stepping in an electrified puddle.
Meanwhile, a woman in Toronto was killed by a falling sign that came apart in high winds.
After days of dire forecasts, warnings and mass evacuations in coastal areas, Sandy came ashore near Atlantic City, N.J., around 8 p.m. ET, but Environment Canada said the storm's effects were being felt as far as 1,000 kilometres away.
The Canadian weather agency said at about 1:30 a.m. ET Tuesday that Sandy was inland over southeastern Pennsylvania, about 120 kilometres west-northwest of Philadelphia. It still had very strong and gusty winds, with sustained winds of 110 km/h.
It was moving west-northwest at 30 km/h. The bulletin said it was expected to weaken "very rapidly" as it moves toward the eastern Great Lakes.
Thousands of flights cancelled
Thousands of flights were cancelled, train service was disrupted, roads were closed and schools and offices were shut down before the storm ever arrived.
Though the exact details of the damage being caused by Sandy are still unclear, the impact is huge: More than 5 million people are in darkness, and a record-breaking four-metre storm surge hit New Jersey and New York City, flooding streets and subway tunnels.
"The West Side Highway is no longer a highway — it's a river with a current," CBC reporter David Common said late Monday night from New York City, noting that water had been reported on roadways in Queens, Brooklyn, Manhattan and beyond.
America's oldest nuclear power plant is also on alert, as officials monitor high water levels at the Oyster Creek plant in Lacey Township, N.J.
They say water levels near Oyster Creek, which is along the Atlantic Ocean, will likely recede within a few hours. Oyster Creek went online in 1969 and provides 9 per cent of New Jersey's electricity.
Storm damage is already projected at $10 billion to $20 billion US, meaning it could prove to be one of the costliest natural disasters in U.S. history.
"There are about 60 blocks in Manhattan now totally without power," Common said late Monday night. "We understand that exists not only across New York City, but right up and down the northeast coast — huge numbers of people without power."
A New York City hospital is moving out more than 200 patients after its backup generator failed when the power was knocked out by the superstorm.
Dozens of ambulances lined up outside NYU Tisch Hospital on Monday night as doctors and nurses began the slow process of taking people out.
In Atlantic City, a popular New Jersey tourist destination, the storm washed away a section of the boardwalk.
"This storm is causing a great deal of damage. Especially on the New Jersey coastline," New Jersey Gov. Chris Christie said on Twitter earlier in the evening.
Christie said he spoke with U.S. President Barack Obama earlier in the day to discuss the state's needs as the storm approached. Christie, an outspoken Republican, said he appreciated Obama's leadership in contacting him.
Christie also expressed concern that some people in coastal areas had not followed orders to evacuate.
In Connecticut, the governor ordered non-essential state employees to stay home from work Tuesday.
According to WBUR, a public radio station in Boston, Sandy brought rough weather and strong surf to some coastal areas in Massachusetts. Hundreds of thousands of people in the state were without power late Monday, part of the sweeping outages that were leaving people in the dark in storm-affected areas.
People in parts of Pennsylvania were also being cautioned about power outages and wild weather.