70 percent of US under storm warning as deep freeze hits

A historic and brutal winter storm put some 240 million Americans under severe weather warnings as the United States faced holiday travel chaos, with thousands of flights cancelled and major highways closed.

Heavy snow and howling winds upended holiday plans for millions at one of the busiest times of the year, just days before Christmas, as a huge cold front swept down from the Arctic and took freezing hold of much of the nation, including normally temperate southern states.

Temperatures plunged below -40 Celsius in some locations, with a staggering 240 million people -- some 72 percent of the US population -- under winter weather warnings or advisories, according to the National Weather Service.

"Winter weather hazards remain in effect from the Canadian border south to the Rio Grande (border river with Mexico), Gulf Coast and central Florida Peninsula while spanning from the Pacific Northwest to the Eastern Seaboard," the NWS said in a report.

The warnings appear to be one of the most sweeping sets of US winter weather advisories ever.

The biting cold is an immediate concern for more than one million customers, mainly in the US south and east, who were without power.

Transportation departments in North and South Dakota, Oklahoma, Iowa and elsewhere reported near-zero visibility, ice-covered roads and blizzard conditions, and strongly urged residents to stay home.

More than 3,520 US flights were already cancelled and another 1,900 delayed.