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.

President Trump is calling for Iran to stop enriching uranium as part of a U.S. proposal on the country's nuclear program. Iran is poised to reject the deal's draft, according to Reuters. CBS News' Natalie Brand has more.