TEHRAN — Iran has started decommissioning the first of thousands of centrifuges used for enriching uranium as part of its commitments under the nuclear deal reached with global powers, the head of Iran’s nuclear energy program was quoted as saying during a visit to Japan on Monday. The head of the Atomic Energy Organization of Iran, Ali Akbar Salehi, told the Kyodo News agency that Iran had started preliminary work, referring to centrifuges and other steps. A group of 20 hard-line lawmakers demanded that the government stop the decommissioning of the centrifuges because a special parliamentary committee to monitor the process sought by Khamenei has not yet been formed.