Police were using helicopters and dogs to search for three convicted murderers who escaped from a northwest Arizona prison, kidnapped two semi-truck drivers at gunpoint and used the big rig to flee.

The death toll from tornados across four US Southern states that tore through homes, ripped the roof off a shopping mall and blew apart warehouses in a rare spasm of violent winter weather rose to at least 21 today, with dozens injured.

A deadly winter storm that caked America’s centre with a thick layer of ice claimed at least 17 lives and blacked out more than 600,000 homes and businesses.

Rival motorcycle gangs armed with guns and knives clashed on a crowded Nevada casino floor yesterday, leaving three dead and at least 12 wounded as terrified gamblers ducked for cover.