He said Iran has the "capacity to produce five percent, twenty percent, sixty percent, or any percentage" of enriched uranium, a claim often repeated by Tehran. Uranium enrichment is the sensitive process that produces fuel for nuclear power plants but also, in highly extended form, the fissile core for a warhead. The current five percent level exceeds the limit set by the accord but is less than the 20 percent Iran had previously operated and far less than the 90 percent level required for a warhead. In its fourth step away from the agreement, Iran resumed enrichment at the Fordow plant south of Tehran on Thursday, with engineers feeding uranium hexafluoride gas (UF6) into the plant's mothballed enrichment centrifuges.