The move is the latest of many by Iran breaching the restrictions imposed by the 2015 nuclear deal, which capped the purity to which Tehran can refine uranium at 3.67%.
In May, the International Atomic Energy Agency reported that Iran was using one cascade, or cluster, of advanced centrifuges to enrich to up to 60% at its above-ground pilot enrichment plant at Natanz. The IAEA informed member states on Tuesday that Iran was now using a second cascade for that purpose, too.
Iran reiterated that its nuclear programme is peaceful and said it had informed the IAEA about its enrichment activities. It added that its moves away from the 2015 deal would be reversed if the United States returned to the accord and lifted sanctions.
"If the other parties return to their obligations under the nuclear accord and Washington fully and verifiably lifts its unilateral and illegal sanctions...all of Iran's mitigation and countermeasures will be reversible," Foreign Ministry spokesman Saeed Khatibzadeh said.