A new burst of arctic air was on the way and there was an “increasing potential of another significant winter storm” for Eastern states this weekend, the National Weather Service said.

People shovel snow along a residential street in the Charlestown neighborhood in Boston, on Monday.Joseph Prezioso / AFP via Getty Images

Extreme cold warnings were in place for 160 million people Tuesday morning, as communities across the eastern third of the United States repaired damage from a huge winter storm that has killed at least 21 people.

The National Weather Service said temperatures would remain well below normal levels for the rest of this week and into the next, with "life-threatening" wind chills dropping conditions to minus 20 degrees Fahrenheit on Monday night in the upper Ohio Valley.

And the cold spell shows no signs of ending, with a new burst of arctic air on the way and "increasing potential of another significant winter storm" for Eastern states this weekend, the weather service said.

This could see the country hit by its lowest temperatures in several years and could be the longest cold spell for several decades.

Snow of at least 12 inches has landed in a huge area stretching from Arkansas to New England, causing widespread disruption, while Pittsburgh got more than 20 inches, as well as temperatures as low as minus 25.

More than 500,000 energy customers were still without power across the country early Tuesday, according to PowerOutage.com, including 178,000 in Tennessee and 140,000 in Mississippi, raising the alarm for vulnerable people who may struggle to heat their homes.

"Record low temperatures tonight across the South and particularly dangerous in the wake of the weekend winter storm with many still without power," the weather service said in an update late Monday.

"Low temperatures in the single digits to below zero pose a life-threatening risk of hypothermia and frostbite even without wind," it continued.

The weather service said another "blast of arctic air" would make its way through the Plains into the East and Southeast on Friday into Saturday, with more record lows expected as far south as Florida.

The cold snap should persist until at least the middle of next week before finally returning to a seasonal average.

While some airports were starting to reopen, there were still more than 1,400 flight cancellations listed by FlightAware on Tuesday morning and more than 8,000 delays.

New York City declared an "enhanced code blue" due to the extreme cold and the city`s Emergency Management Department would open more than 50 hospitals and shelters to provide warmth for people who need it. About half a million students in New York logged in for remote learning Monday as schools remained closed.

In Dallas, about 1,150 homeless people were in a shelter provided by the city, as well as 10 households and 14 pets.

The severe weather has put huge pressure on emergency services who have faced high levels of callouts and carried out widespread rescues. State troopers in New Hampshire responded to 126 crashes since the storm began.

In Pittsburgh, there was almost an inch of snow per hour Monday, making it the second-biggest 24-hour snow accumulation in 30 years. Mayor Corey O`Connor declared a state of emergency and said some 37 snowplows broke down in the city because of the extreme cold.

A state trooper in Louisiana caught the moment a tree fell onto a road in Natchitoches parish, illustrating how trees and power lines are weighed down by the extreme cold, causing widespread disruption and damage.

Adrian Ronca-Hohn told The Associated Press that about 40 trees collapsed around his property in northeast Mississippi, meaning he and his family will have to chainsaw their way out. "I mean, it looks like a war zone out here," he said. "We couldn`t go 10 seconds without hearing what sounded like a gunshot."

Mississippi Gov. Tate Reeves said he would deploy 500 National Guard troops by Wednesday to help remove debris and manage traffic. "We will get through this," he said. "We`re not going to get through it today and we`re not going to get through it tomorrow."

The city of Monroe, Louisiana, issued a precautionary warning to boil water for a minute before drinking it because the weather had caused it to have "questionable microbiological quality" due to "temporary operational challenges."