Some 500,000 households are without power in the United States as a huge winter storm stretches from the southern Rocky Mountains in the west of the country to New England on the northeast coast.

Heavy snow, sleet and freezing rain are threatening almost 180 million people - more than half the population - the National Weather Service said. A total of 37 states are affected.

"The snow and ice will be very, very slow to melt and won't be going away anytime soon," said meteorologist Allison Santorelli.

Josh Weiss, a meteorologist at the National Oceanic and Atmospheric Administration, said there are two aspects that make this storm "unique": its broad spread, covering 2,000 miles of the country, and the extreme cold it is forecast to leave behind.

Across the country, more than 13,000 flights were cancelled on Saturday and Sunday, according to flight tracking website FlightAware.

Sunday's cancellations are already the most on any single day since the COVID pandemic, said aviation analytics firm Cirium.

At least 17 states and the District of Columbia have declared emergencies, said homeland security secretary Kristi Noem.

"Stay home if possible," she advised.

Mikie Sherrill, the governor of New Jersey, said she was expecting conditions the "likes of which we haven't seen in years".

Announcing a 35mph speed limit on highways, she added: "It's a good weekend to stay indoors."

About half a million power outages were reported in the path of the winter storm, including more than 100,000 each in Mississippi, Texas, and Tennessee, according to PowerOutage.us.

In Louisiana, there were reports of vehicles hitting fallen trees and branches falling onto houses in the parish of DeSoto.

In Georgia, senior state meteorologist Will Lanxton said it was "perhaps the biggest ice storm we have expected in more than a decade".

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In New York City, thousands of bin lorries had been fitted with snow ploughs, mayor Zohran Mamdani said.

This weekend, "each 12-hour shift will be staffed by more than 2,000 sanitation workers", he said in a video posted on X.

Temperatures will be at their coldest for eight years, he added.