A historic winter weather system that brought bitter, record-busting cold to much of the southern and central United States was pushing up the East Coast on Thursday, with forecasters warning of heavy snowfall and dangerous, icy buildups.

The frigid blast has over the past week seen Arctic cold envelope a US heartland unfamiliar with such extremes, leaving dozens of dead in its wake and millions of people in oil-rich Texas without power.

The National Weather Service (NWS) said a "major" winter storm would impact an area stretching from Virginia up to the Northeast. 

"The swath of heaviest snowfall is forecast to occur from the Appalachians of Virginia and West Virginia to northern Maryland and southern Pennsylvania," the NWS said, warning of treacherous travel conditions. 

Across Texas, which has been hardest hit by the cold snap, utility companies were gradually restoring power though more than 500,000 homes and businesses remained without electricity Thursday morning.

Texas power companies had to implement rolling blackouts to avoid grids being overloaded as residents cranked up the heat. The surge in demand came just as generating capacity drooped thanks to power stations and wind turbines freezing.

David Hernandez, 38, spent the night at a Houston church with other people who had fled their homes.

"My car got stranded and I was trying to sleep in the car but it was just too cold," Hernandez said. "Liquids in my car were actually turning to ice so it was like sleeping in an ice box.

"I had to come here," he said. "There's no choice."

Texas authorities have opened about 300 emergency "warming centers" across the state.

Compounding the misery, thousands of Houston residents were suffering from both power outages and a loss of water pressure.

Nearly seven million Texans were being advised to boil their water before drinking it or using it for cooking, said Toby Baker, who heads the Texas Commission on Environmental Quality, adding that nearly 264,000 people were impacted by non-operational water systems.

Texas's woes have sparked outrage in the Lone Star State, the only one of the US's 48 continental states to have its own independent power grid.

Beto O'Rourke, a former Democratic presidential candidate from Texas, said it was at risk of becoming "a failed state." 

Vaccination sites shut

Even though the Arctic air mass was beginning to loosen its grip in Texas and elsewhere in the south, the NWS said frigid temperatures would continue.

"Record cold daily maximum and minimum temperatures are likely to transpire in the South Central US through Saturday morning," it said.

More than 30 storm-related deaths have been reported by US media since the cold weather arrived last week, many in traffic accidents.

President Joe Biden was forced to postpone until Friday a visit to the Pfizer Covid-19 vaccine manufacturing site in Kalamazoo, Michigan while federal government offices in Washington were closed Thursday.

Jeff Zients, the White House Coronavirus Response Coordinator, said the cold weather was impacting delivery and distribution of Covid-19 vaccines.

"There's certain parts of the country, Texas being one of them, where vaccination sites are understandably closed," Zients said. 

"What we're encouraging governors and other partners to do is to extend hours once they're able to reopen."

Houston police said a woman and a girl died from carbon monoxide poisoning after sitting in a car in a garage with the engine running to keep warm.

And emergency medical authorities around Texas said dozens of others have been treated for suspected carbon monoxide poisoning, while 77 were treated for hypothermia in northern Texas on Tuesday.

A dozen animals -- including one 58-year-old female chimpanzee -- died during the freeze at rescue sanctuary Primarily Primates near San Antonio, the organization said on its website.

The winter storm has spawned at least four tornadoes, according to Atlanta-based weather.com, including one in North Carolina on Monday that killed at least three people and injured 10.

Across the southern border, Mexican officials said six people died after temperatures plunged.