The UN nuclear watchdog issued a warning to Tehran on Wednesday after it found that changes had been made without prior notification at the Fordo Fuel Enrichment Plant to equipment that can enrich uranium to up to 60 percent.

Iran claimed that an IAEA inspector had accidentally flagged the changes as being undeclared, and that the matter was later resolved.

"Iranian claims that this action was carried out in error are inadequate," the joint statement said.

"We judge Iran's actions based on the impartial and objective reports of the IAEA, not Iran's purported intent."

According to the IAEA report, seen by AFP, during an unannounced Fordo inspection on January 21, inspectors 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 IAEA 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," their statement said.

The Fordo site has been under increased scrutiny since Iran began producing uranium enriched to 60 percent there since November 2022, as well as at its Natanz site.

That far exceeds the 3.67 percent enrichment threshold set by the 2015 agreement between Tehran and major powers, and is close to the 90 percent needed to produce an atomic bomb.