The US state of Oklahoma is reviewing its execution procedures after an inmate was left writhing in agony following a lethal injection.

Clayton Lockett, 46, was sentenced to die by lethal injection on Tuesday night, but his execution was dramatically halted after he was seen moving and mumbling 13 minutes after the drugs should have rendered him unconscious and killed him.

After 43 minutes of thrashing around in agony and his vein “exploding” after he was injected, Lockett finally died of a massive heart attack.

Lockett had been sentenced to death for the shooting of a 19-year-old woman and watching his friends bury her alive.

Fellow Oklahoma death row inmate Charles Warner, who was due to die by lethal injection shortly after Lockett, has been given a two week stay of execution as the authorities investigate the incident and review their procedures.

Both prisoners had been involved in a court battle with the state of Oklahoma over the drugs to be used in their executions, but Oklahoma’s High Court found that the men had no right to know the source of the drugs used to kill them.