"There are nearly 4,000 centrifuges working in the Natanz enrichment facility... another 3,000 centrifuges are being installed," IRNA quoted Alireza Sheikh Attar as saying in an interview with state television.

In July, President Mahmoud Ahmadinejad said Iran had up to 6,000 centrifuges for uranium enrichment, the process at the heart of Western fears that Tehran is secretly trying to build nuclear weapons.

The International Atomic Energy Agency, which has been probing Iran's nuclear activities for several years, said in May that Tehran was operating 3,500 centrifuges in Natanz, a huge underground complex in central Iran.

Iran is under three sets of UN Security Council sanctions over its refusal to freeze enrichment and risks further sanctions for failing to give a clear response to an incentives package offered by six world powers in return for a halt to the sensitive work.

World powers offered to start pre-negotiations with Iran during which Tehran would add no more uranium-enriching centrifuges and in return face no further sanctions.

Under the enrichment process, low-grade uranium is refined into fuel that can power reactors, or at highly enriched levels, into weapons-grade material.

Iran, a leading OPEC oil producer, denies seeking nuclear weapons and insists its programme is designed to provide energy for its growing population when its reserves of fossil fuels run out.

Permanent Security Council members Britain, China, France, Russia and the United States plus Germany made its offer to Iran, which included trade incentives and help with a civilian nuclear programme, in June.

The United States, which has been pushing for tougher sanctions, has never ruled out a military option over the nuclear standoff, but is for the moment pursuing a diplomatic route.

Iranian officials have repeatedly said they have no intention of freezing enrichment and that as a signatory to the nuclear Non-Proliferation Treaty, Iran has the right to make its own nuclear fuel.

IAEA chief Mohamed ElBaradei is due to submit another report on Iran's nuclear programme and its cooperation with the UN nuclear watchdog agency in mid-September.

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