Cities are all but shutting down across the US Midwest as the region shivers in a deadly cold snap known as a polar vortex.

At least seven people have been killed in several states as a result of the arctic weather.

Temperatures fell to -30C in Chicago – colder than parts of Antarctica – and -37C in North Dakota.

Freezing weather will chill 250 million Americans, and 90 million will experience -17C or below.