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 US has sent Iran a proposal for a nuclear deal between Tehran and Washington, the White House confirmed on Saturday. Iranian Foreign Minister Abbas Araghchi said he had been presented with "elements of a US deal" by his Omani counterpart Badr Albusaidi during a short visit to the Iranian capital.