By Francois Murphy and John Irish VIENNA (Reuters) -U.N. inspectors monitoring Iran's Fordow nuclear site confronted a major gap in their knowledge last year as they watched trucks carrying advanced uranium-enriching centrifuges roll into the facility dug into a mountain south of Tehran.

Iran has warned the U.S. that it will not give up its right to its enrich uranium, casting doubt on prospects for a deal as Tehran considers a U.S. proposal for curbing its nuclear program. Newsweek has reached out to the U.