The Atomic Energy Organisation of Iran unveiled advanced centrifuges at Natanz
Iran will begin enriching uranium at its underground Fordo facility on Wednesday, rolling back another commitment under a 2015 nuclear deal.
President Hassan Rouhani said the step was reversible if world powers party to the accord upheld their commitments.
Enriched uranium can be used to make reactor fuel but also nuclear weapons.
This is the fourth step taken by Iran since July in response to sanctions the US reinstated when abandoning the deal.
The 2015 accord was intended to curb Iran’s nuclear ambitions in return for relief from sanctions but President Donald Trump unilaterally pulled out of it in May 2018.
Mr Trump wants to force Iran to negotiate a new agreement that would place indefinite curbs on its nuclear programme and also halt its development of ballistic missiles. But Iran has so far refused.
The other parties to the deal – the UK, France, Germany, China and Russia – have tried to keep it alive. But the sanctions have caused Iran’s oil exports to collapse, the value of its currency to plummet, and sent its inflation rate soaring.
Iran has insisted that its nuclear programme is entirely peaceful.
Before 2015, the country had two enrichment facilities – Natanz and Fordo – where uranium hexafluoride gas was fed into centrifuges to separate out the most fissile isotope, U-235.
The deal saw Iran agree to only produce low-enriched uranium, which has a 3-4% concentration of U-235, and can be used to produce fuel for nuclear power plants. Weapons-grade uranium is 90% enriched or more.