Harris County Judge Lina Hidalgo, the top government executive in Harris County, Texas, on Wednesday said the ongoing storms were straining not only the local electric grid but triggering a cascade of effects, including lost water pressure, carbon monoxide poisoning and halted COVID-19 vaccinations.

But she said much of what the state is dealing with is a "man-made disaster" stemming from how the electricity grid is managed by the Electric Reliability Council of Texas (ERCOT).

The worst USA power outages by far have been in Texas, where officials requested 60 generators from the Federal Emergency Management Agency and planned to prioritise hospitals and nursing homes. "Tell me what's going on". While it's true that some of Texas' windmills weren't properly prepared for a deep freeze, others were still overproducing power early in the story. She said residents statewide deserve answers.

In the southeast, a low-pressure system that developed along the arctic front created fuel for storms that unleashed at least four tornadoes, said meteorologist Jeremy Grams of the weather service's Storm Prediction Center in Norman, Oklahoma.

Snow and record cold continued Wednesday in the southwestern state where temperatures are forecast to remain at or below freezing.

ERCOT, which instituted blackouts to cope with a surge in demand it says was caused by the unusually cold weather, said it was working to restore power as quickly as possible.

Austin Energy, the local power company, said almost 200,000 area customers were without electricity. The company used rolling blackouts Tuesday in and around Oklahoma City despite below-zero temperatures.

The Southwest Power Pool, a group of utilities covering 14 states, said the blackouts were "a last resort to preserve the reliability of the electric system as a whole".

Besides the rolling blackouts in Texas, there are more than 200,000 without power in the Appalachian states.

More than 20 storm-related deaths have been reported in the United States since the cold weather arrived last week, a lot of them in traffic accidents.

The tornado that hit North Carolina's Brunswick County was an EF3 with winds estimated at 160 miles per hour (257 kph), the weather service said on Twitter.

At least 20 people have died, some while struggling to find warmth inside their homes.

Sharon Benson, 63, said her roof was damaged and her garage door blown off. Windows were shattered and nearby trees were uprooted.

A vehicle is stuck on a snow-covered road in Austin on Wednesday. A MS man died after losing control of his vehicle, which overturned on an icy road Monday night near Starkville, Oktibbeha County coroner Michael Hunt said Tuesday.

Authorities said a fire that killed three young children and their grandmother in the Houston area was likely to have spread from the fireplace they were using to keep warm. In Oregon, authorities confirmed Tuesday that four people died in the Portland area of carbon monoxide poisoning.

A man in Louisiana died when he hit his head after slipping on ice, and a 10-year-old Tennessee boy perished after he and his six-year-old sister fell through the ice into a pond Sunday.

Four died in Monterrey, three of them homeless people who succumbed to exposure and one person who died at home from carbon monoxide poisoning from a heater.

Several cities had record lows: In Minnesota, the Hibbing/Chisholm weather station registered minus 38 degrees Fahrenheit (minus 39 degrees Celsius).

At midday, more than 2,700 USA flights had been canceled, led by more than 800 at Dallas Fort Worth International Airport and more than 700 at Bush Intercontinental in Houston. About 100 school systems closed, delayed opening or switched to remote classes in Alabama, where forecasters said conditions might not improve until temperatures rise above freezing Wednesday afternoon.

Millions of Texans still don't have power days after the state's independent energy grid failed, leading to the deaths of several people in the state.