Iran has invited the UN nuclear agency to visit the heavy water plant linked to a still unfinished reactor that could produce enough plutonium for up to two warheads a year.
Yukiya Amano, who heads the UN's International Atomic Energy Agency, said the IAEA will accept the offer to visit the plant in the central city of Arak, which comes less than a week after a landmark nuclear deal between Iran and six world powers.
He also said his agency first learned that it would be tasked with supervising Iranian compliance with that deal as an agreement was struck over the weekend.
Mr Amano didn't give a date for when the IAEA would start implementing its role under the Geneva deal, but suggested that it would take some time, in part because his agency wasn't informed earlier to prepare for the mission.
The invitation for December 8 is not part of the six-power deal, which commits Iran to freeze its nuclear programme for six months in return for limited relief from economic sanctions. But it shows Tehran is starting to comply with separate commitments to open previously off-limits sites to IAEA inspectors.
The status of the Arak plant had been one of the major issues during negotiations leading to last weekend's agreement in Geneva.
Iranian Foreign Minister Mohammad Javad Zarif said on Wednesday some construction will continue at Arak. When completed, Arak could produce plutonium which could be used for nuclear weapons, although Iran insists its programme is entirely for peaceful purposes such as producing electricity, and for scientific and medical research.
Iran had scheduled completion for next year, a timetable described by experts as too ambitious.
US State Department spokeswoman Jen Psaki said Mr Zarif's comments didn't constitute a violation of the agreement, even though Iran effectively pledged to freeze advancement at the facility.
IAEA employees have had some access to the reactor, 250 kilometres (150 miles) southwest of Tehran. But they haven't been able to inspect the plant on the same site since 2011. Heavy water helps control nuclear activity of the fuel rods used in some reactor types.
Beyond commitments on the Arak reactor under the Geneva nuclear deal, Iran also agreed to limit uranium enrichment. It pledged to stop enriching uranium to a stage that is only a technical step away from concentration needed to arm nuclear warheads and to keep its stockpile of lower enriched uranium from expanding.
A senior Iranian official said on Thursday that Iran would increase its low-level uranium enrichment. Iranian nuclear chief Ali Akhbar Salehi said centrifuges previously used for higher-level enrichment would now be turned to produce low-enriched uranium.