VIENNA (Reuters) - World powers are demanding that Iran immediately mothball a uranium enrichment site it hid for years, heightening fears it is secretly planning to build atom bombs, in a resolution drafted by the U.N. nuclear watchdog. It voiced "serious concern" -- a diplomatic euphemism for alarm -- over its cover-up of the Fordow project and said it was in blatant breach of U.N. demands for an enrichment suspension. But IAEA chief Mohamed ElBaradei suggested to Reuters in an interview on Wednesday the new resolution could backfire by aggravating Iran's siege mentality, boosting nuclear hardliners.