Millions of Americans across the country are on alert for a severe weather outbreak as violent, long-track tornadoes with damaging winds of up to 80 mph and large hail are expected across the Midwest and South with the cross-country storm moving east.

At least three people have died as a powerful system of storms sweeps across a wide swathe of the central and southern of the United States, with another three killed in vehicle crashes as blinding winds whipped through Texas and Oklahoma.