VIENNA (Reuters) — Iran has started enriching uranium at its underground Natanz plant with a second type of advanced centrifuge, the IR-4, IAEA said in a report reviewed by Reuters on Tuesday, in a further breach of Tehran’s deal with major powers.

Last year Iran started moving three cascades, or clusters, of different advanced models of centrifuge from an above-ground plant at Natanz to its below-ground Fuel Enrichment Plant (FEP). It is already enriching underground with IR-2m centrifuges. The nuclear deal only lets it enrich there with first-generation IR-1 machines.

However, Iran has indicated that it now plans to install a second cascade of IR-4 centrifuges at the FEP but installation of that cascade has yet to begin, the report said. Iran has already increased the number of IR-2m machines, which are far more efficient than the IR-1, installed at the underground plant.