VIENNA: Iran has started enriching uranium with one of three cascades, or clusters, of advanced IR-6 centrifuges recently installed at its underground enrichment plant at Natanz, a report by the UN atomic watchdog to member states seen by Reuters said on Monday. Iran is using the cascade of up to 174 machines to enrich uranium to up to 5% purity, the confidential report said. Of the other two IR-6 cascades at the underground plant, one was undergoing passivation, a process that precedes enrichment, and the other had not yet been fed with nuclear material, it added.