VIENNA (Reuters) - Iran has started the process of enriching uranium to 20% purity with advanced centrifuges at its Fordow facility buried inside a mountain, the U.N. nuclear watchdog said on Wednesday, a move likely to raise tensions at talks on Iran's 2015 nuclear deal.

       The International Atomic Energy Agency (IAEA) verified on Tuesday that Iran fed uranium hexafluoride feedstock enriched to up to 5% into a cascade, or cluster, of 166 IR-6 centrifuges at Fordow to enrich it further to up to 20%, the IAEA said in a statement. An IAEA report last month said Iran was operating 166 IR-6 machines there without keeping the enriched product.

       (Reporting by Francois Murphy; Editing by Alex Richardson)