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's stance directly clashes with U.S. President Donald Trump 's position that Iran must halt all enrichment amid negotiations marked by uncertainty, despite having reached their most significant step since they began in April with a U.S. offer being delivered to Tehran last week.

Iran’s supreme leader effectively rejected a U.S. proposal that allows temporary uranium enrichment in the country before ending it completely, forcing the White House to reconsider its approach as tensions mount over Iran’s nuclear program.