A UN watchdog report shows Iran is being inconsistent in meeting its nuclear obligations, the United States, Britain, France and Germany said in a joint statement on Friday.
The International Atomic Energy Agency (IAEA) criticised Iran for making an undeclared change to the interconnection between clusters of machines enriching uranium to up to 60 per cent purity, close to weapons grade, at its Fordow plant.
Iran said the IAEA's position on Tehran's nuclear work was not correct.
The IAEA found the change during an unannounced inspection on January 21 at the Fordow Fuel enrichment Plant (FFEP), a site dug into a mountain where inspectors are stepping up checks after Iran said it would dramatically expand enrichment.
"As stated by the Agency, this unnotified change is inconsistent with Iran's obligations under its NPT-required Comprehensive Safeguards Agreement," the four countries said in their statement, referring to the Treaty on the Non-Proliferation of Nuclear Weapons.
"The IAEA inspector's interpretation was incorrect but he reported it to the agency ... We immediately provided the explanation to the IAEA on the same day," Iranian nuclear chief Mohammad Eslami said on Thursday.
The production of high-enriched uranium by Iran at Fordow carries significant proliferation-related risks and is without any credible civilian justification, the joint statement said, adding Iran has not offered a credible answer yet to the IAEA's outstanding questions as part of its safeguards investigation.
Fordow is so sensitive that the 2015 nuclear deal between Iran and major powers banned enrichment there. Since the United States pulled out of the deal in 2018 and reimposed sanctions against Iran, the Islamic Republic has breached many of the deal's restrictions on its nuclear activities.