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.

A senior Israeli official says Israel believes deeply buried stocks of enriched uranium at one Iranian nuclear facility hit by the U.S. military are potentially retrievable.

The process of uranium enrichment increases the concentration of U-235, an isotope that can sustain a nuclear chain reaction. Beginning in 2006, the U.N. Security Council called on Iran to suspend all uranium enrichment, but it endorsed the JCPOA on July 20, 2015.