At least 21 people have died in one of the worst cold snaps to hit the US Midwest in decades.

Ninety million people ‑ a third of the US ‑ have seen temperatures of ‑17C or below.

Some 250 million Americans overall have experienced the “polar vortex” conditions.

Hospitals have been treating patients reporting frostbite as parts of the country ground to a halt.

Temperatures are expected to swing to above average over the weekend.

[Source: BBC ]