The blast of Arctic cold that hit the United States Midwest this week will linger on Thursday, but it is pushing east and losing much of its bite after causing at least a dozen deaths.

Schools were closed for Wednesday and Thursday across the Midwest, including Chicago, and police warned of the heightened risk of accidents on icy highways.

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

Illinois State Police officers rescued 21 people stranded in a charter bus that broke down in sub-zero temperatures along Interstate 55 near Auburn after the vehicle's diesel fuel turned to gel in its engine.

More than a thousand flights, close to two-thirds of those scheduled, were cancelled on Wednesday into or out of Chicago O'Hare and Chicago Midway international airports.

The bitter 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.