VIENNA (Reuters) - Iran has accelerated its enrichment of uranium to near weapons-grade, switching to two clusters of advanced centrifuges enriching up to 60% purity from one, the U.N. nuclear watchdog said in a report on Tuesday seen by Reuters.

       Where before Iran was using one cascade of 164 IR-6 centrifuges to enrich up to 60% at an above-ground plant at Natanz, the International Atomic Energy Agency verified that it was now using that cascade and another of 153 IR-4 machines for the same purpose, the IAEA said in the report to member states.

       (Reporting by Francois Murphy; Editing by Alex Richardson)