“Iran‘s letter to the agency … did not say when this enrichment activity would take place,” the IAEA statement said.

Fordow was built inside a mountain, apparently to protect it from aerial bombardment, and the 2015 deal does not allow enrichment there. Iran is already enriching at Fordow with first-generation IR-1 centrifuges.

Iran has breached the deal’s 3.67% limit on the purity to which it can enrich uranium, but it has only gone up to 4.5% so far, well short of the 20% it achieved before the deal and the 90% that is weapons-grade.

The deal’s main aim was to extend the time Iran would need to produce enough fissile material for a nuclear bomb, if it chose to, to at least a year from roughly two to three months. It also lifted international sanctions against Tehran.