|Iran has fiercely defended its right to develop nuclear technology for what it says are peaceful purposes [Reuters]
The UN atomic agency has said that Iran is now enriching uranium to 20 per cent purity at a hard-to-bomb mountain bunker in Fordo.
Iran had admitted the existence of the previously secret facility in 2009 and earlier International Atomic Energy Agency (IAEA) reports said that Iranian scientists were preparing to begin operating the facility's centrifuges.
IAEA spokeswoman Gill Tudor said on Monday that all nuclear material "remains under the agency's containment and surveillance" at Fordo.
But the fear is if Iran decided to expel IAEA inspectors and enrich uranium to weapons-grade purity of 90 per cent, Fordo would enable them to produce enough fissile material in a short space of time.
"This clearly represents an escalation," Mark Hibbs from the Carnegie Endowment for International Peace think-tank told AFP news agency.
Experts point out that the process of obtaining 20 per cent enriched uranium represents most of the work needed to get the uranium enriched to the level of 90 per cent or above required for atomic weapons.
Fordo, a reinforced facility sunk deep under a mountain 150km southwest of Tehran, is designed to be a difficult if not impossible target to bomb.
The beginning of enrichment operations at the Fordo facility, 150km southwest of Tehran, was reported by Iran's Kayhan daily newspaper on Sunday. It is small in comparison to Iran's main enrichment site in Natanz in central Iran, where nearly 8,000 centrifuges are operating.
Enrichment at Natanz began in 2006. But the centrifuges at Fordo are considered more efficient and are shielded from aerial surveillance and protected against airstrikes by up to 90m of mountain rock.
Iran has been hit by four rounds of UN sanctions and the US and the EU have imposed increasingly tight economic sanctions on Tehran over its nuclear programme.
It has remained defiant. In a televised speech on Monday, Supreme Leader Ayatollah Ali Khamenei said: "Sanctions imposed on Iran by our enemies will not have any impact on our nation. The Iranian nation believes in its rulers."