Brutally cold temperatures gripped the US Midwest on Thursday, freezing water mains, causing power outages, canceling flights and straining natural gas supplies.

Tens of millions of Americans shivered for a second day as the mercury dipped to record lows in several states.

More than a dozen deaths have been attributed to the sub-zero weather and a weekend snowstorm that blanketed the same region. Many of the cold-related fatalities were in Michigan, where the governor said the death toll was still being confirmed.

Schools and businesses remained closed in several midwestern states, people were encouraged to stay home, and travelers were stranded by grounded flights and halted trains.

In Michigan and Minnesota, natural gas supplies were under threat. Authorities asked residents to reduce heat consumption wherever possible and decreased heating in government buildings.

Water mains froze in Detroit, Chicago and parts of Canada, and power outages were reported in Iowa and Wisconsin.

Nearly 1,700 flights had been canceled in Chicago by Thursday afternoon. Airport crews worked in 15-minute increments on the tarmac to avoid frostbite.

Rail service Amtrak planned to begin partially restoring service after canceling all lines Wednesday in and out of Chicago.

The deadly, sub-zero temperatures were expected to lift Friday, but the misery would not end quickly in the roughly dozen states most affected.

"We are not done yet. We've got another 24 hours where the weather will be at dangerous levels," Michigan Governor Gretchen Whitmer told a news conference.

The National Weather Service (NWS) said temperatures would slowly moderate, but the agency forecast wind chills Thursday would remain between -20 to -50 degrees Fahrenheit (-29 to -46 Celsius) over parts of the Upper Midwest.

The cold has frozen sections of Niagara Falls and sent blocks of ice floating down the river winding through downtown Chicago.

- More snow to come -

The Arctic air mass that descended from its usual northern rotation on Wednesday caused the second coldest day ever recorded in the Windy City, where residents reported hearing "frost quakes."

Local television station WGN said booms heard by residents were likely from the frozen, water-saturated ground cracking under their feet.

The NWS said a low temperature of -21F (-29C) was recorded in Chicago on Thursday morning. The record low of -27F (-33C) was on January 20, 1985.

It said a historic low temperature for Illinois of -38F (-39C) had been reported in the town of Mt Carroll and was being reviewed before being declared a state record.

Record low temperatures were also reported in some towns in Iowa and Wisconsin.

Officials in multiple states warned that the extreme weather should be taken seriously, with the risk of hypothermia and frostbite setting in within minutes of exposure.

There were also concerns over another round of snow late Thursday, after a weekend snowstorm inundated the areas now frozen by Arctic cold.

"We've gone from snow to freezing temperatures, wind chill," Chicago Mayor Rahm Emanuel said at a news conference. "With more snow on the way, we're not out of the woods yet."

Hundreds of warming centers were opened for vulnerable residents such as seniors, and shelter capacities increased for the homeless.

Among the dead was an 18-year-old University of Iowa student.

He was found unresponsive behind a campus building Wednesday morning, when wind chill temperatures in Iowa City were -51F (-46C), according to local TV station KCCI.

- Calls for conservation -

While officials warned residents to remain on guard as long as the sub-zero weather persisted, authorities in Michigan and Minnesota were also asking them to turn down their thermostats to conserve natural gas.

Supplies were strained due to high demand from home heaters and from a fire at a natural gas compressor station in Michigan, officials said.

"This is not over until noon tomorrow. And we are asking people to continue to keep the thermostat down," Michigan Governor Whitmer said.

General Motors and Fiat Chrysler suspended or curtailed operations at more than a dozen facilities in Michigan to conserve natural gas, the companies said.

City crews braved sub-zero weather to repair a number of frozen water main breaks that plagued Motor City neighborhoods.

Chicago reported 22 broken water mains, 16 of which had already been repaired.

America's northern neighbor Canada was also contending with extreme cold, with frozen water pipes, snarled travel on a major waterway, and temperatures as low as -40F (-40C) on Wednesday.