A massive winter storm headed towards the northeast United States on Sunday after sweeping across much of the country, threatening tens of millions of Americans with blackouts, transportation chaos and bone-chilling cold.

After battering the country's southwest and central areas, the storm system began to hit the heavily populated mid-Atlantic and northeastern states with snow and freezing rain as a frigid air mass settled in across the nation.

"The snow/sleet impacts will linger well into next week with rounds of re-freezing that keeps surfaces icy and dangerous to both drive and walk on for the foreseeable future," the National Weather Service (NWS) said.

Snowfall was reported across the central United States, including Kansas, Oklahoma and Missouri, where some spots already recorded eight inches on the ground by Saturday night, the NWS said.

Shoppers stripped supermarket shelves as the weather service forecast huge snowfall in some areas and possibly "catastrophic" ice accumulations.

Transportation Secretary Sean Duffy warned that up to 240 million Americans could be affected by the storm.

At least 20 states and the US capital Washington have declared states of emergency.

"Take this storm seriously, folks," the NWS said on X, predicting an "astonishingly long swath" of snowfall from Colorado to Maine.

About 14,000 flights in and out of the United States had been canceled over the weekend, with thousands more delayed, according to tracker FlightAware.

Speaking at the Washington headquarters of the US Federal Emergency Management Agency, Department of Homeland Security chief Kristi Noem urged Americans impacted by the weather to "be smart, stay home if possible, take care of your family members, check on your neighbours and continue to work with your local officials."

US President Donald Trump, who was riding out the storm at the White House, said on his Truth Social platform: "We will continue to monitor, and stay in touch with all States in the path of this storm. Stay Safe, and Stay Warm!"

The brutal storm system is the result of a stretched polar vortex, an Arctic region of cold, low-pressure air that normally forms a relatively compact, circular system but sometimes morphs into a more oval shape, sending cold air spilling across North America.

Scientists say the increasing frequency of such disruptions of the polar vortex may be linked to climate change, though the debate is not settled and natural variability plays a role.

The NWS warned that heavy ice could cause "long-duration power outages, extensive tree damage, and extremely dangerous or impassable travel conditions," including in many states less accustomed to intense winter weather. (AFP)