UPDATE 4-U.S. polar vortex blamed for at least 21 deaths

(Recasts to reflect higher death tolls, adds deaths in Iowa, Ohio)

Jan 31 (Reuters) - Tens of millions of Americans braved Arctic-like temperatures as low as minus 56 degrees Fahrenheit (minus 49 Celsius) on Thursday that paralyzed the U.S. Midwest and were blamed for at least 21 deaths.

Warmer weather was on the way, but that offered little comfort to vulnerable populations such as the homeless and elderly enduring cold that caused frostbite in minutes and made being outside potentially deadly.

Officials across multiple states linked numerous deaths to the frigid air. The death toll rose from a previous 12 after at least nine more people in Chicago were reported to have died from cold-related injuries, according to Dr Stathis Poulakidas at the city's John H Stroger Jr Hospital.

A University of Iowa student was found dead on campus of possible exposure early on Wednesday, according to university officials. The wind chill at the time police found Gerald Belz, 18, was minus 51 F (minus 46 C), according to the National Weather Service.

Homeless and displaced people were particularly at risk, with Chicago and other cities setting up warming shelters. But many toughed it out in encampments or vacant buildings. A 60-year-old woman found dead in an abandoned house in Lorain, Ohio, was believed to have died of hypothermia, Lorain County Coroner Stephen Evans said.

"Theres just no way if youre not near a heat source that you can survive for very long out in weather like this," Evans told the Chronicle-Telegram newspaper.

It has been more than 20 years since a similar Arctic blast covered a swath of the Midwest and Northeast, according to the weather service.

More than 30 record lows were shattered across the Midwest. Cotton, Minnesota, had the lowest national temperature recorded early on Thursday at minus 56 F (minus 48 C) before the weather warmed up, the weather service reported.

U.S. homes and businesses used record amounts of natural gas for heating on Wednesday, according to preliminary results from financial data provider Refinitiv.

In Detroit, General Motors Co suspended operations at 11 Michigan plants after a utility made an emergency appeal to conserve natural gas. Fiat Chrysler Automobiles NV also canceled a shift on Thursday at two of its plants.

Relatively balmier weather was on the horizon. By the weekend, Chicago was expected to bask in snow-melting highs in the mid-40s to low 50s Fahrenheit, along with other parts of the Midwest, the weather service said.

(Reporting by Jonathan Allen in New York; additional reporting by Rich McKay in Atlanta, Suzannah Gonzales and Karen Pierog in Chicago, Gina Cherelus in New York, and Katharine Jackson in Washington Writing by Andrew Hay Editing by Bill Tarrant and Matthew Lewis)