Tens of millions of people in the United States are battling under a potentially deadly arctic chill, with rail tracks set alight to keep trains running and officials warning against the risk of frostbite risks for those stepping outside.

Companies have told their workers to stay home, schools are closed and hundreds of flights have been cancelled.

America's National Weather Service has warned people who are exposed to the extreme cold are susceptible to frostbite "in a matter of minutes".

The US Postal Service - known for its commitment to bringing the mail whatever the weather - has even reportedly suspended deliveries in Iowa due to the severe cold.

Chicago, America's third largest city, was expected to be colder than parts of Antarctica.

The extreme weather conditions prompted a number of rail companies to cancel all trains in and out of the city for Wednesday and Thursday. On Tuesday, crews from north east Illinois's Metra Commuter Rail used a gas-fed system to set rail tracks alight in Chicago in order to keep trains moving.