A dangerous winter storm has swept across the US, leaving at least three people dead and cutting power to hundreds of thousands of homes.

Schools and roads across the country have been closed, and flights have been cancelled as "life-threatening" conditions stretched from Texas to New England, according to the National Weather Service (NWS).

At least two people died of hypothermia in Louisiana, with state health officials linking their deaths to the storm, and another death was reported in Texas.

As of Sunday afternoon, about 900,000 households had lost power, according to poweroutage.us. Meanwhile, more than 10,000 flights were cancelled, FlightAware reported.

Widespread heavy snow, sleet and freezing rain, which is a dangerous phenomenon where cooled rain droplets freeze instantly on surfaces, could last for days and could affect around 180 million Americans - more than half the population.

"The snow and the ice will be very, very slow to melt and won't be going away anytime soon, and that's going to hinder any recovery efforts," Allison Santorelli, a meteorologist with the National Weather Service, told the BBC's US media partner CBS News.

Louisiana's Department of Health confirmed on Sunday that two men had died of hypothermia.

The mayor of Austin, Texas, said there had been an "exposure-related" death.

New York City Mayor Zohran Mamdani wrote in a post on X that at least five people in the city had died on Saturday, but added their cause of death was yet to be determined.

He said, however, "It is a reminder that every year New Yorkers succumb to the cold".

New York State Governor Kathy Hochul warned residents to stay inside and off roads.

"This is certainly the coldest weather we've seen, the coldest winter storm we've seen in years," she said on Sunday.

"A sort of arctic siege has taken over our state and many other states across the nation."

Hochul said the "brutal" conditions were expected to bring the longest cold stretch and the highest snowfalls in years.

"It is bone-chilling, and it is dangerous," she said.

Kentucky Governor Andy Beshear said on Sunday that the state was seeing more ice and less snow than was originally predicted.

"That is not good news for Kentucky," he said.

Weather experts have warned that one of the biggest dangers of the storm is ice, which can damage trees, down power lines, and make roads unsafe.

More than 200 car crashes were reported in Virginia as the storm moved into the state, according to local media.

Emergencies declared

Nearly half the states have declared emergencies, and schools across the country are already cancelling classes in anticipation of the storm continuing into Monday. The US Senate has also scrapped a scheduled vote for Monday evening.

In declaring an emergency in the nation's capital, Washington Mayor Muriel Bowser said: "We're experiencing the biggest snowstorm in a decade in DC this weekend."

While places in the north, such as the Dakotas and Minnesota, are used to below-freezing temperatures in winter, it is unusual to see such extreme cold in states like Texas, Louisiana, and Tennessee, where temperatures are around 15-20 °C below the seasonal average.

Those states could also see ice accretions of about an inch from freezing rain.

The polar vortex - a ring of strong westerly winds that form above the Arctic every winter, containing a pool of very cold air - led to the powerful storm, according to weather experts.

When the winds are strong, they stay in place, when the winds weaken, the vortex loops further south and cold air plunges toward the US. As the cold air meets mild air in the south, the air rises and storm fronts form.

In this case, the winter storm is pushing northwards and eastwards, clearing the Canadian maritime by Tuesday but leaving more cold air in its wake. It is forecast to stay dangerously cold into the start of February.

Some experts contend climate change could influence the behaviour of the polar vortex due to changes in sea surface temperatures in our warming world.