Iran has escalated its uranium enrichment with advanced machines at its underground Fordow plant, the UN atomic watchdog, IAEA, said in a report on Saturday.
Western diplomats have long expressed concern about devices this cascade, or cluster, of centrifuges is equipped with.
The use of these so-called modified sub-headers means Iran could switch more quickly and easily to enriching to higher purity levels.
Iran’s move came ten days after indirect talks with the United States in Doha with the mediation of the European Union failed. The US said that Tehran did not come to the meeting in a serious negotiating posture and had simply repeated “extraneous” demands.
While Iran is required to inform the International Atomic Energy Agency about such a switch, if it chose not to, it might escape detection for some time as there is currently a lag between Iran's enrichment and IAEA inspectors' verification of what is produced.
"On 7 July 2022, Iran informed the Agency that, on the same day, it had begun feeding the aforementioned cascade with UF6 enriched up to 5% U-235," the confidential report to IAEA member states said. Reuters reported on the development after seeing the IAEA report.
UF6 refers to uranium hexafluoride gas which is fed into centrifuges to be enriched.
In a report on June 20 also seen by Reuters, the IAEA said that months after Iran informed it of its intention to use the cascade, Iran had begun feeding UF6 into it for passivation, a process that comes before enrichment.
The IAEA verified on July 6 that passivation had ended, Saturday's report said.
"On 9 July 2022, the Agency verified that Iran had begun feeding UF6 enriched up to 5% U-235 into the cascade of 166 IR-6 centrifuges with modified sub-headers for the declared purpose of producing UF6 enriched up to 20% U-235," it said.
Iran is already enriching to up to 60% elsewhere, well above the up to 20% it produced before its 2015 deal with major powers that capped its enrichment level at 3.67% but still below the roughly 90% of weapons grade.
The move is the latest step of many to breach and move well beyond the restrictions which the 2015 deal imposed on Iran's nuclear activities. It comes as talks to revive that deal are at an impasse and Western powers have warned time is running out to reach an agreement.
An agreement was said to have been close in March, after 11 months of talks in Vienna, but the negotiations stopped and remained stalled. The agreement would have removed major US economic sanctions, but Iran insisted that sanctions not directly related to the nuclear dispute should also be removed. On major cluster of sanctions target the Revolutionary Guard, but these are related to its role in terrorism and regional destabilization.
The United States pulled out of the 2015 nuclear agreement known as the JCPOA in 2018 under then-President Donald Trump, re-imposing sanctions against Tehran that the deal had lifted.
A year later, Iran began retaliating by breaching the deal's restrictions on the level of uranium enrichment. Tehran further escalated enrichment as Joe Biden won the US presidential election and embarked on talks with Iran and other JCPOA signatories to revive the agreement.
Now, Iran has accumulated enough 60-percent enriched uranium to be able to take the next step of enriching the fissile material to 90 percent purity needed for one nuclear bomb.
With reporting by Reuters