VIENNA, Nov 11 (Reuters) - Iran has installed a first cascade of advanced centrifuges in the underground Natanz uranium enrichment plant that its deal with major powers says can only be used for first-generation IR-1 machines, a report by the U.N. atomic watchdog said.

Iran had previously informed the International Atomic Energy Agency that it would transfer three cascades of the uranium-enriching machines from an above-ground pilot plant at the Natanz nuclear site to the underground one after an above-ground centrifuge workshop burned down in an apparent act of sabotage.

Iran has installed and connected the cascade of IR-2m centrifuges but has not fed the cascade with uranium hexafluoride gas, the feedstock for centrifuges, the confidential IAEA report, obtained by Reuters, said on Wednesday.

(Reporting by Francois Murphy Editing by Mark Heinrich)