US Defense Secretary James Mattis has rejected Iranian accusations the US was behind Saturday’s attack on a military parade in southern Iran, which killed 29 and injured 60 people, many of them civilians. "It would be good if they knew what they were talking about,” Mattis told reporters at the Pentagon on Monday, adding that "it’s ludicrous to allege that we might have had anything to do with it.” Tehran has blamed the US and its Middle Eastern allies of being behind the Ahvaz attackers, and vowed a “crushing and devastating” response. Iranian authorities have already arrested more than two dozen suspects.