Iran's nuclear program is irreversible, President Mahmoud Ahmadinejad said on Wednesday, voicing continued defiance in the face of possible new international sanctions on the Islamic Republic.

He also reiterated that Iran now has 3,000 centrifuges, used to enrich uranium, in its underground Natanz nuclear plant. Enriched uranium can fuel power plants but also, if refined further, provide material for bombs.

Western experts say 3,000 machines running smoothly for long periods at supersonic speed could make enough enriched uranium for an atomic bomb in about a year, if Iran wanted, and form the basis for industrial-scale nuclear fuel production.

The Iranian nation has entered the phase of industrial scale of nuclear fuel (production) and the train of the Iranian nation's progress is irreversible, Ahmadinejad told a rally in South Khorasan province broadcast live on state television.

Today, we've reached 3,000 centrifuges, he said.

Ahmadinejad, who like other Iranian leaders rejects U.S. accusations the country is seeking to develop atom bombs, said in September that Iran had more than 3,000 centrifuges working and that more were installed every week.

But an International Atomic Energy Agency report said in August Iran remained well short of 3,000 centrifuges functioning and that its rate of enrichment was still far below capacity.

Iran now appears to have nearly 3,000 installed, divided into 18 cascades of 164 each, diplomats and analysts said, but there is still no evidence they are being run together, or that all are being fed with uranium for enrichment.

Iran likely has managed to learn how to operate individual cascades adequately but may still be struggling to operate a large number at the same time in parallel, analysts David Albright and Jacqueline Shire wrote in the November issue of Arms Control Today.

...Iran is likely to need several more months to get (3,000 centrifuges) fully operational.

Six world powers agreed in London last week to push ahead with a third round of U.N. sanctions against Iran unless reports this month indicate Tehran has tried to address their concerns about its atomic activities.

They passed resolutions based on wrong information, Ahmadinejad said, referring to two previous U.N. sanctions votes. The Iranian nation will never back down, even one iota.

Britain, France, Germany, the United States, Russia and China are expected to meet again on November 19 to assess reports from IAEA head Mohamed ElBaradei and European Union foreign policy chief Javier Solana.

The United States imposed unilateral economic sanctions on Iran last month and has not ruled out military action against Iran if diplomacy fails to halt Iran's sensitive atomic work.

But diplomats from other countries have said they want to hear how Iran's talks with the IAEA are progressing.

We are for negotiation but we would never negotiate over our independence and rights, Ahmadinejad said.

(Additional reporting by Mark Heinrich in Vienna; editing by Dominic Evans)