Polar vortex in pictures: Millions hit by icy blast

Tens of millions of people across the United States are experiencing a blast of cold weather that has already claimed at least six lives.

With temperatures dropping to once-in-a-generation lows, here is a selection of some of the best photographs:

As the sun came up in Chicago, cold steam rose from Lake Michigan

A state of emergency has been declared in the Midwestern states of Wisconsin, Michigan and Illinois.

Other states not quite so used to cold temperatures, including Mississippi and Alabama, are also dealing with tough conditions.

Forecasters predict that Chicago, one of the country's busiest cities, could get colder than Antarctica in the coming days.

Commuters dressed up to the nines to face the sub-zero temperatures at rush hour

The Chicago River has frozen over, a sight rarely seen by locals

Thousands of flights were cancelled on Tuesday and further travel disruption is expected.

Warming shelters were open overnight as police searched for people most at risk from cold conditions.

Frostbite is possible within 10 minutes of exposure to the extreme weather, the National Weather Service has warned.

Schools have been closed amid plummeting conditions, including in Minneapolis, Minnesota

Thick icicles encase a tree on Lake Michigan in Chicago

Tess McNamara, from Minneapolis, had to bundle up in an outfit more suited to skiing on Tuesday

A thermometer in Minneapolis, Minnesota, shows almost -30F (-37C)

Schools in several states have announced blanket closures.

"If you don't have to be outside, don't," meteorologist Amy Seeley warned Chicago residents.

An atmospheric, wintery scene met residents of Chicago on Wednesday morning

The strong snow has pleased some, including nine-year-old Kamaire Styles from Colorado

The Cloud Gate sculpture reflects the freezing scene of Chicago

Forecasters say the Arctic front will start moving across New York on Wednesday

All photographs subject to copyright.