The five men who carried out twin attacks in Tehran were Iranians who joined the Islamic State group in Iraq and Syria before returning last summer, the intelligence ministry said on Thursday.

"The five known terrorists… after joining the Daesh (IS) terrorist group, left the country and participated in crimes carried out by this terrorist group in Mosul and Raqa," the ministry said in a statement.

The statement indicates only five people carried out Wednesday's twin attacks, rather than six originally reported.