- Iran nuclear deal
image copyrightAFPimage captionEuropean officials said they had "grave concern" about the move
Iran has begun the process of producing enriched uranium metal, which could help it develop a nuclear weapon, the UN's atomic watchdog has said.
Tehran informed the International Atomic Energy Agency (IAEA), and said the process was to develop fuel for a research reactor.
British, French and German officials said the move could threaten talks to revive the abandoned 2015 nuclear deal.
The US called it an "unfortunate step backwards".
The deal, known formally as the Joint Comprehensive Plan of Action (JCPOA), imposed restrictions on Iran's nuclear programme to make it harder for them to develop nuclear weapons.
In return, the US and European signatories agreed to lift economic sanctions that were in place.
Former President Donald Trump pulled the US out of the deal in 2018 and reinstated sanctions against Iran, after which Tehran began violating many of its restrictions.
Current US President Joe Biden's administration has so far retained Mr Trump's sanctions against Iran.
Now, negotiators from the US and Europe have been holding talks in Vienna to try and restore the agreement.
image copyrightReutersimage captionThe talks were adjourned on 20 June
The talks began in April and were adjourned on 20 June, with no date set for the next round.
Iran's President-elect Ibrahim Raisi wants the US to lift sanctions on his country in exchange for complying with the deal.
In a statement on Tuesday, the IAEA said: "Today, Iran informed the Agency that UO2 (uranium oxide) enriched up to 20% U-235 would be shipped to the R&D (research and development) laboratory at the Fuel Fabrication Plant in Esfahan, where it would be converted to UF4 (uranium tetrafluoride) and then to uranium metal enriched to 20% U-235, before using it to manufacture the fuel."
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Officials from the UK, France and Germany said that they had "grave concerns" about Iran's decision.
"Iran has no credible civilian need for uranium metal R&D and production, which are a key step in the development of a nuclear weapon," the three countries said in a joint statement issued by the British Foreign Office.
"With its latest steps, Iran is threatening a successful outcome to the Vienna talks despite the progress achieved in six rounds of negotiations."
The statement also urged Iran to return to the talks in the Austrian capital.
US State Department spokesperson Ned Price said that although they were not setting a deadline for the talks, "as time proceeds Iran's nuclear advances will have a bearing on our view of returning to the JCPOA".
media captionFrom 2018: What was the Iran nuclear deal?