Iran has started moving the machines that enrich uranium for nuclear fuel from its main atomic complex in the central city of Natanz to an underground bunker near the holy city of Qom, its top nuclear official was quoted as saying on Monday.
"Transferring Natanz centrifuges to Fordow (near Qom) is under way with full observance of standards," Fereydoun Abbasi-Davani told state broadcaster IRIB, adding "Fordow's facilities are being prepared and some centrifuges have been transferred."
Iran announced in June that it would shift its production of higher-grade uranium to the underground site at Fordow, in defiance of international calls on Tehran to halt uranium enrichment which some countries say is aimed at developing nuclear bombs, a charge Iran denies.
Iran only disclosed the existence of Fordow to the UN nuclear watchdog in September of 2009 after learning that Western intelligence agencies had detected the mountain site.
Moving sensitive nuclear work to the underground bunker could offer greater protection against any attacks by Israel or the United States, which have both said they do not rule out pre-emptive strikes to stop Iran getting nuclear weapons.
Tehran said in June that it aimed to triple its capacity to enrich uranium to a higher grade -- 20 percent fissile purity -- which it says will be used to power a medical research reactor.
Iran says it is now manufacturing nuclear fuel plates out of the 20 percent material for that purpose, although Western officials and analysts suspect Tehran's actual goal is to further enrich to the 90 percent level required for atom bombs.
A day after the June announcement, the six world powers that have negotiated with Tehran about its nuclear program issued a new statement about their "deepened concerns" of possible military dimensions to its atomic activity.
There was no immediate comment from the United Nations' International Atomic Energy Agency (IAEA) on Abbasi's comments.
In an interview with Reuters on Friday, IAEA Director General Yukiya Amano said that moving centrifuges to Qom and increasing enrichment capacity was a "further deviation" from several UN Security Council resolutions, which demand that Iran suspend all enrichment-related activities to foster serious negotiations on a peaceful solution to the dispute.
When asked whether Iran was installing centrifuges at Fordow, Amano told Reuters that his agency "would know better in coming weeks", referring to the latest quarterly report on Iran's nuclear activities due next month.
Amano said the IAEA was in talks with Iran on how the agency's inspectors would monitor activities at Fordow.
"What we are doing is that we are monitoring and we are negotiating the new safeguard approach to verify the activities," he said. "They are having close contact with us to agree on the new safeguard procedure."
Last week Russia launched a fresh diplomatic campaign to re-engage Iran in nuclear talks with world powers that stalled in January over Iran's insistence on its right to enrich uranium despite UN resolutions calling on it to stop.