The UN atomic agency confirmed Monday that Iran has started enriching uranium at a new site in a difficult-to-bomb mountain bunker, in a move set to further heighten tensions with the West, AFP reported.
"The IAEA can confirm that Iran has started the production of uranium enriched up to 20 percent... in the Fordo Fuel Enrichment Plant," International Atomic Energy Agency spokeswoman Gill Tudor said in a statement.
"All nuclear material in the facility remains under the Agency's containment and surveillance," the Vienna-based watchdog added.
The Islamic republic admitted the existence of the facility in 2009 and earlier reports from the IAEA had said that Iranian scientists were preparing the facility to begin operations.
Iran, which insists its nuclear programme is for exclusively peaceful purposes, has repeatedly said it will not abandon uranium enrichment despite four rounds of UN Security Council resolutions calling on Tehran to desist.
While nuclear energy plants need fuel enriched to 3.5 percent, Iran says the 20-percent enriched uranium is needed for its Tehran research reactor to make isotopes to treat cancers.
Western powers, however, reject this, believing Iran has been researching ways to develop and deliver nuclear weapons, and has piled on sanction after sanction to try to halt the work.
Experts point out that the process of obtaining 20 percent enriched uranium represents most of the work needed to get the uranium enriched to the level of 90 percent or above required for atomic weapons.
Creating such fissile material is one of three main areas needed to develop a nuclear arsenal. Iran would also need to make the enriched uranium weapons-ready and manufacture a missile to carry it to target.
A report from the IAEA in November, the agency's hardest-hitting to date, included a trove of evidence that Western powers said confirmed Iranian efforts in these other two areas.
The United States, the European Union and other allies imposed tighter sanctions after the report, which Iran dismissed as "baseless".
A Western diplomat at the IAEA's headquarters in Vienna told AFP that the development "serves only to increase the international community's serious concerns."
Another diplomat said it showed Iran was "completely ignoring United Nations Security Council resolutions."
"I don't think this crosses a red line for other member states. But it certainly sets a worrying trend... You have to ask, when Iran says it is considering engaging (with the international community) and when it says its programme is peaceful, why they are putting something in a bunker?"
Iran's IAEA envoy, Ali Asghar Soltanieh, was not immediately available for comment.
Fordo, a reinforced facility sunk deep under a mountain 150 kilometres (90 miles) southwest of Tehran, is designed to be a difficult if not impossible target to bomb.
Natanz, which is also a hardened target though more vulnerable than Fordo, is already operating with some 8,000 centrifuges working to enrich uranium.
Iran's supreme leader Ayatollah Ali Khamenei said that Tehran will not bow in the face of the sanctions, in comments broadcast on state television on Monday.
"The firm decision of the Islamic Republic is to resist the pressures" of Western governments, Khamenei said.
"While the Iranian people have travelled the road to success and see the signs of new victories to come, the (Western) oppressor is trying to frighten the Iranian people and officials by brandishing the threat of sanctions."