The story has affected millions of people in as many as 20 states. Photo: BBC

Across the typically mild South, more than half a million homes and businesses lack power, and more than 3,300 US flights have been cancelled.

The mammoth storm has affected people in 22 states from Texas to Maine and caused at least 10 deaths.

The most crowded swath of the US - between Washington DC and Boston - is bracing for up to 8in (20cm) of snow.

Even a snowplough found itself in tough driving conditions in North Carolina

A band stretching from North-eastern Pennsylvania through New York State's Hudson Valley and into New England could see 10-20in of snow on Thursday, the National Weather Service warned.

The storm, described by the National Weather Service as an event of "historical proportions", leaves in its southern wake a wreckage of snapped tree limbs and power lines coated in as much as 1in (2.54cm) of ice, motorways turned to car parks, road accidents, and residents shivering in darkened homes.

Forecasters said it was one of the worst storms to strike Atlanta, the largest city in the South, since 1973.

President Barack Obama offered the might of the US federal government in aid, declaring a disaster in the state of South Carolina and all northern counties in Georgia.

On Wednesday evening, the Federal Emergency Management Agency (Fema) said it was moving supplies, including generators, meals, water, blankets and cots to an emergency centre in Atlanta.

Heavy ice brought down trees and power lines in Georgia

At least 10 deaths have already been blamed on the storm, including three people killed when an ambulance slid off an icy Texas road and caught fire and a man in Georgia who slipped and fell on a patch of ice.

Thousands of vehicles are backed up on snow-covered motorways around Raleigh, North Carolina, with some people abandoning their vehicles.

Soo Keith, of Raleigh, left her office shortly after mid-day, but after two hours had only driven a few miles.

Ms Keith told the Associated Press news agency she eventually abandoned her vehicle and continued on foot, arriving home four hours later.

"My face is all frozen, my glasses are all frozen, my hair is all frozen," she said.

"I know how to drive in the snow. But this storm came on suddenly and everyone was leaving work at the same time. I don't think anybody did anything wrong; the weather just hit quickly."

As much as one inch of ice coated some parts of the South

Residents of Georgia appeared to have heeded official warnings, with motorways in the state clear but with many people stuck at home without electric power.

"Thanks to the people of Georgia," Governor Nathan Deal said. "You have shown your character."

Mr Deal told those waiting for power to be restored to "be patient", saying he was hearing of "good response times" from the state's power companies.

The bad weather caused a run on grocery items, including milk, in Georgia

The storm was described by official forecasters as "catastrophic", "crippling" and "paralysing"

Residents took to walking in Chattanooga, Tennessee

And schools were pre-emptively closed across much of the South