After keeping away inspectors for two years, Iran is inviting the UN nuclear agency to a facility linked to a still unfinished reactor that could produce enough plutonium for up to two warheads a year.

The invitation to visit the heavy water plant in the central city of Arak on Dec. 8 comes less than a week after a landmark nuclear deal between Iran and six world powers to freeze its nuclear program for six months in return for limited relief from economic sanctions.

The invite is not part of the six-power deal. But it shows Tehran is starting to comply with separate commitments to open previously off-limits sites to inspectors of the UN's International Atomic Energy Agency.

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 Wednesday some construction will continue at Arak. When completed, Arak could produce plutonium which could be used for nuclear weapons, although Iran insists its program is entirely for peaceful purposes such as producing electricity, and for scientific and medical research.

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IAEA staff have not been able to inspect Iran's heavy water nuclear facility near the central city of Arak since 2011. (Hamid Foroutan/ISNA/Associated Press)

Iran had scheduled completion for next year, a timetable described by experts as too ambitious.

U.S. State Department spokeswoman Jen Psaki said Zarif's comments didn't constitute a violation of the agreement, even though Iran effectively pledged to freeze advancement at the facility.

Iran's decision to allow inspectors to visit the Arak heavy water plant was announced by IAEA chief Yukiya Amano to the agency's 35-nation board. It will enable inspectors to get a clearer picture of how far work has advanced at that site.

IAEA employees have had some access to the reactor, 250 kilometres 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.

Iran says it will increase low-level uranium enrichment

A senior Iranian official said Thursday that Iran would increase its low-level uranium enrichment. Iranian nuclear chief Ali Akhbar Salehi told the IRNA news agency that centrifuges previously used for higher-level enrichment would now be turned to produce low-enriched uranium.

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Ali Akbar Salehi, head of Iran's nuclear department, says machines that were producing 20 per cent enriched uranium will be engaged in producing low-level enriched uranium. (Hanz Punz/Associated Press)

He didn't elaborate. But the Geneva deal doesn't prohibit Iran from making more enriched uranium. It stipulates only that all newly-produced material must be turned into oxide, which is difficult to reconvert.

Amano's agency will expand its monitoring of Iran as part of the Geneva deal to make sure Tehran lives up to its commitments.

Diplomats told The Associated Press on Wednesday the agency probably won't be in a position to start on that role until January. While giving no dates, Amano indirectly confirmed some delays, telling the meeting Thursday that implementation will "take some time."