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.

Iranian, Egyptian and U.N. leaders met in Cairo on Monday to discuss Iran’s nuclear program after the U.N. nuclear watchdog agency said Iran is further increasing its stockpile of uranium enriched to near weapons-grade levels.