The UN human rights office has said it is "deeply troubled" by four executions over eight days in Arkansas under an accelerated timetable based on the looming expiration of a sedative used in the procedure.

Arkansas has executed its fourth inmate in eight days, concluding a schedule of lethal injections that was set to beat the expiration date of one of the drugs.

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.