They say Iran must abandon uranium enrichment, but this is one of our red lines. If talks are conditional on halting enrichment, they simply won’t happen," said Ali Velayati, senior adviser to Supreme

Officials in Israel are acknowledging that some enriched uranium may have survived the powerful U.S. strikes on Iranian nuclear facilities last month.

The assessment came as experts are trying to determine how long it would take Iran to rebuild its nuclear program in the aftermath of U.S. and Israeli strikes.