The condemnation followed the latest report by the International Atomic Energy Agency (IAEA), the UN’s atomic watchdog, which determined that Iran has started up new cascades of advanced centrifuges and plans to install others in the coming weeks after facing criticism over its nuclear program.
That report came a day after the IAEA said that Iran has rapidly installed extra uranium-enriching centrifuges at its Fordow site.
"Iran has taken further steps in hollowing out the JCPoA, by operating dozens of additional advanced centrifuges at the Natanz enrichment site as well as announcing it will install thousands more centrifuges at both its Fordow and Natanz sites," a joint statement by the three countries said, referring to the Joint Comprehensive Plan of Action signed with Iran in 2015.
"This decision is a further escalation of Iran’s nuclear program, which carries significant proliferation risks," it added.
The joint statement stressed that "Iran’s decision to substantially increase its production capacity at the underground Fordow facility is especially concerning".
"Iran is legally obliged under the Non-Proliferation Treaty to fully implement its safeguards agreement, which is separate to the JCPoA."
The latest IAEA reports followed a resolution passed by the IAEA Board of Governors censuring Iran over its lack of cooperation with the IAEA. Diplomats said Iran’s latest activity was as limited retaliation to the resolution.
The IAEA has long sought answers from Iran on uranium traces which were found at undeclared sites. While the number of sites under investigation has been narrowed to two from three, Iran still has yet to give the IAEA satisfactory answers on how the traces got there.
The IAEA released a report at the end of May which found that Iran has further increased its stockpile of uranium enriched to near weapons-grade levels.
Previously, the IAEA found that, between June and November last year, Iran slowed down the enrichment to 3 kg per month, but that jumped back up to a rate of 9 kg at the end of the year.
The increase came soon after Tehran barred a third of the IAEA's core inspections team, including the most experienced, from taking part in agreed monitoring of the enrichment process.
That move was part of Iran’s scaling back of its compliance with the 2015 nuclear deal it signed with world powers, in response to then-US President Donald Trump’s withdrawal from the deal in 2018.