A blast of Arctic air brought the coldest temperatures felt in two decades to the central US today, causing at least four deaths, forcing businesses and schools to close and cancelling thousands of flights.  

Shelters for the homeless were overflowing.

The severe cold has been described by some meteorologists as the "polar vortex" and dubbed by media as the "polar pig".

Temperatures were 11-22C below average in parts of Montana, North and South Dakota, Minnesota, Iowa, Wisconsin, Michigan and Nebraska, according to the National Weather Service.

More than half the flights at Chicago's O'Hare International Airport were closed as fuel supplies froze, leaving crews unable to fill aircraft tanks.

The afternoon temperature in Chicago was -12C, making it colder than in areas of Antarctica, parts of which hovered around 0C during the southern hemisphere summer.

The Arctic air was moving toward the east coast where temperatures were expected to fall throughout the day as low as -18C in some areas tomorrow.

The coldest temperatures in years and gusty winds were expected as far south as Brownsville, Texas and central Florida, the National Weather Service said.

The northeast saw unseasonably mild weather and rain, but authorities warned travellers to expect icy roads and sidewalks tomorrow.

At least four weather-related deaths were reported.

In northwest Missouri, a one-year-old boy died when the vehicle he was riding in crashed into a snow plough on an icy highway, according to the Missouri State Highway Patrol.             

In Indianapolis, firefighters found the body of an elderly woman outside her home early this morning.

"It appeared she had been there for a while," said Captain Michael Pruitt of the Wayne Township Fire Department.

A 58-year-old man died yesterday in northeastern Oklahoma after he lost control of his car on icy roads, the Oklahoma Highway Patrol said.

In Chicago, a 48-year-old man died of a heart attack while shovelling snow last night.

In Cleveland, Ohio, where the temperature was -19C and was forecast to drop to -21C overnight, homeless shelters were operating at full capacity.

Shelter operators had begun to open overflow facilities to accommodate more than 2,000 people who had come seeking warmth.

"There are also going to be people that won't go into the shelters," said Brian Davis, an organiser with Northeast Ohio Coalition for the Homeless.

Frostbite can set in within minutes in such low temperatures, according to experts.

The National Weather Service issued warnings for life-threatening wind chills in western and central North Dakota, with temperatures as low as -51C.

The US cold snap mirrored or outdid freezing weather inparts of the world as Almaty, Kazakhstan where it was -22C, Mongolia, where temperatures reached -23C, and Irkutsk, in Siberia, where it was -33C.

Over 4,300 flights were cancelled and 3,577 delayed, according to FlightAware.com, which tracks airline activity.

Many airlines could not allow their ground crews to remain outdoors for more than 15 minutes at a time.

There were hundreds of cancellations by airlines including United, Southwest, and American.

The bitter cold combined with blowing snow was complicating rail traffic as well. Union Pacific, one of the largest railroads and a chief mover of grains, chemicals, coal and automotive parts, warned customers that the weather was causing delays up to 48 hours across Illinois, Iowa, Minnesota, Missouri and Wisconsin.