VIENNA: Iran is rapidly expanding its ability to enrich uranium with advanced centrifuges at its underground plant at Natanz and now intends to go further than previously planned, a confidential UN nuclear watchdog report seen by Reuters showed on Monday. While indirect talks between Iran and the United States on reviving the 2015 Iran nuclear deal have stalled, Tehran has brought onstream an ever larger number of advanced centrifuges the deal bans it from using to produce enriched uranium. These machines are far more efficient than the first-generation IR-1, the only centrifuge that the deal lets Iran use to grow its stock of enriched uranium. Iran has been adding them particularly at two underground sites at Natanz and Fordow that may be designed to withstand potential aerial bombardment. The third of three cascades, or clusters, of advanced IR-6 centrifuges recently installed at the underground Fuel Enrichment Plant (FEP) at Natanz has now come onstream, said the International Atomic Energy Agency report to member states. Diplomats say the IR-6 is Iran’s most advanced centrifuge.