A blast of icy polar air brought dangerously low temperatures to the U.S. Midwest on Wednesday, causing at least three more deaths while halting mail delivery and forcing residents to huddle indoors.

At least 12 deaths related to extreme cold weather have been reported since Saturday in Michigan, Iowa, Indiana, Illinois, Wisconsin and Minnesota, according to officials and media reports.

In Chicago and the Pennsylvanian city people were seen walking outside, bundled up and dressed for sub-zero temperatures, in the painfully cold air as temperatures hovered around minus 28 degrees Celsius.

Frigid winds began to blow into the U.S. East Coast later on Wednesday, sending temperatures plunging.

In Batavia, New York, 21 vehicles, including a state police vehicle, were involved in a major crash, according to local media reports.

The cold was caused by a displacement of the polar vortex, a stream of air that normally spins around the stratosphere over the North Pole but whose current was disrupted and was now pushing south.

Officials opened warming centers across the Midwest, and in Chicago police stations were open to anyone seeking refuge. Five city buses were also deployed to serve as mobile warming centers for homeless people, while city police handed out hats, jackets and blankets.