The US, France, Britain and Germany have condemned Iran's "inadequate" response to a report by a UN watchdog, which raised new concerns about the country's nuclear programme.
The International Atomic Energy Agency on Wednesday said it found substantial changes had been made without prior notification at Iran's Fordow plant, to equipment that can enrich uranium to up to 60 per cent.
Iran claimed that an IAEA inspector had accidentally flagged the changes as being undeclared, and that the matter was later resolved.
But the western powers said Tehran's "claims that this action was carried out in error are inadequate".
"We judge Iran's actions based on the impartial and objective reports of the IAEA, not Iran's purported intent," their joint statement said.
According to the IAEA report, an unannounced inspection of the Fordow plant on January 21 found that "two IR-6 centrifuge cascades were interconnected in a way that was substantially different from the mode of operation declared by Iran to the agency".
The watchdog did not specify the kind of changes made to the interconnection between the cascades.
The four countries said that the change was "inconsistent with Iran's obligations" under treaties and that "such lack of required notifications undermines the Agency's ability to maintain timely detection at Iran's nuclear facilities".
"We recall that the production of high-enriched uranium by Iran at the Fordow Enrichment Plant carries significant proliferation-related risks and is without any credible civilian justification," they said.
The Fordow site has been under increased scrutiny since Iran began producing uranium enriched to 60 per cent there since November 2022, as well as at its Natanz site.
That far exceeds the 3.67 per cent enrichment threshold set by the 2015 agreement between Tehran and major powers, and is close to the 90 per cent needed to produce an atomic bomb.