A November 2019 photo released by Iran's Atomic Energy Organisation shows the interior of the Fordow Uranium Conversion Facility. — AFP Photo

VIENNA — The UN's nuclear watchdog said on Monday that Iran's stock of heavy water for reactors has surpassed the limit set under its agreement with world powers.

The International Atomic Energy Agency (IAEA) said in a statement that Iran's heavy water production plant was in operation and that its stock of heavy water reserves was 131.5 tonnes, above the 130-tonne limit.

In Vienna, an IAEA spokesperson said: "On 17 November, the Agency verified that the Heavy Water Production Plant (HWPP) was in operation and that Iran’s stock of heavy water was 131.5 metric tonnes."

Heavy water is not itself radioactive but is used in nuclear reactors to absorb neutrons from nuclear fission.

Heavy water reactors can be used to produce plutonium for nuclear weapons as an alternative to enriched uranium.

It was the first time the agency has recorded a volume greater than the level agreed upon as part of the Joint Comprehensive Plan of Action (JCPOA) reached in 2015 with Britain, China, France, Germany, Russia, the United States and the European Union.

The US unilaterally withdrew from it last year, after which Iran began reducing its commitments in a bid to win concessions from those still party to the accord.

In Washington on Monday, Secretary of State Mike Pompeo said the US will lift sanctions waivers on Iran's Fordow nuclear plant, citing the resumption of uranium enrichment activities at the site already announced by Tehran.

"The United States will terminate the sanctions waiver related to the nuclear facility at Fordow effective December 15, 2019," Pompeo told a news conference.

Earlier this month, the IAEA said that uranium particles had been detected at an undeclared site in Iran.

The report also confirmed that Iran has ramped up uranium enrichment in breach of the 2015 deal, feeding uranium hexafluoride gas into previously mothballed enrichment centrifuges at Fordow, an underground plant south of Tehran.

That allows for the production of the most fissile isotope, Uranium 235.

Since September, Iran has also been producing enriched uranium at a facility in Natanz.

It has exceeded a 300 kilogramme limit on stocks of enriched uranium and has breached a uranium enrichment cap of 3.67 per cent.

Iran has always insisted that its nuclear programme is exclusively peaceful and that acquiring nuclear weapons would be contrary to Islamic principles. — AFP