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.

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.

The preliminary U.S. proposal came as a confidential U.N. report described an Iranian initiative that had multiplied Tehran’s stockpile of near-bomb-grade uranium.