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.

U.N. inspectors have faced significant challenges in tracking Iran's nuclear activities, particularly since the U.S. withdrawal from the 2015 nuclear deal. As new talks seek to impose restrictions, closing knowledge gaps about Iran's centrifuges and uranium enrichment is vital for a successful agreement.

Iran has dramatically increased its stockpile of uranium needed to create nuclear weapons and has fallen short of compliance with international regulators, according to an IAEA report.