"A second cascade (of centrifuges) has been activated in continuation of Iran's research activities and within the framework of the Non-Proliferation Treaty," Foreign Ministry spokesman Mohammad Ali Hosseini told reporters.
Iran insists it is enriching uranium to make electricity, but the West says it believes Iran plans to make nuclear weapons.
Hosseini also urged all parties concerned with the nuclear dispute to continue the path of negotiations as the best way of resolving the problem, Iran's Islamic Republic News Agency reported.
A coalition at the United Nations led by the United States wants to impose economic sanctions on Iran for its continued program, but Russia and China have opposed the effort.
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