USA Correspondent: 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 (0F) 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.

Who are the victims?

Homeless people have been particularly at risk, with warming shelters set up across cities.

But some still braved the freezing conditions and one woman, aged 60, was found dead in an abandoned house in Lorain, Ohio.

A hospital in Chicago has already treated 50 patients for frostbite, and some may end up losing a limb, CNN reports. Half of those patients were homeless individuals , while others had jobs that required them to be outdoors.

Some people were found dead a short walk from their homes:

A Michigan man who froze to death in his neighbourhood had been "inadequately dressed for the weather", officials said

In a wind chill of -46C (-51F) an 18-year-old student was found unresponsive a short walk from his dorm on Wednesday and later died in hospital

On Tuesday, a man froze to death in a garage in Milwaukee, Wisconsin, having "apparently collapsed after shovelling snow", according to a medical examiner

Dangerous roads have also been a factor in the deaths. A man was fatally struck by a snow plough near Chicago on Monday and in northern Indiana, a 22-year-old police officer and his wife died after a collision on icy roads.

What's the forecast?

The icy cold is expected to loosen its grip on Friday.

By the end of the weekend, Chicago could see temperatures as high as 10C (50F).

"It's going to be at least a 60-degree swing for Chicago," David Hamrick, a National Weather Service forecaster, told Reuters news agency.

The sudden weather change coming this weekend may be the fastest warm-up on record, meteorologists say.

But as the temperatures abruptly turn warmer, US emergency officials warn of flooding and utility risks.

Pipes can burst with such temperature fluctuations, and rapidly melting snow and ice could cause flooding, the Federal Emergency Management Agency cautioned.