"We have told the (International Atomic Energy) Agency that we are studying and conducting research on different types of machines. We cannot limit ourselves when we have an enrichment programme," said Mohammad Saidi, the vice president of Iran's Atomic Energy Organisation.

"But when it comes to which type we will use, we are still examining this. It isn't the P-2 (centrifuge) -- there are other devices that are more advanced and that are a part of our work," he added.

Centrifuges are used to enrich uranium for either nuclear reactor fuel or atom bomb material. They work in cascades of hundreds, or thousands, spinning at high speed to refine out the uranium U-235 isotope.

Enrichment is seen as a "breakout capacity" which, once mastered, makes manufacturing nuclear weapons possible.

Tehran says its nuclear programme is a peaceful effort to generate electricity, while the United States charges it is hiding covert work on atomic weapons.

Iran announced earlier this month that it had successfully enriched uranium to reactor-grade levels using less advanced P-1 centrifuges, defying a UN Security Council demand for the work to halt while suspicions remain.

But the more advanced P-2 centrifuge can enrich at a much faster rate and is considered far more effective than the P-1 in the production of weapons-grade material.

Iran is suspected of acquiring P-2 devices on the black market network of disgraced Pakistani nuclear scientist Abdul Qadeer Khan, the father of the country's atom bomb.

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