The head of the UN’s nuclear watchdog has warned that Iran is enriching uranium at purity levels that “only countries making bombs are reaching”.

Rafael Grossi, the director-general of the International Atomic Energy Agency, told the Financial Times the situation was “very concerning” as Iran’s nuclear programme had become more sophisticated over the past two years.

“A country enriching at 60 per cent is a very serious thing — only countries making bombs are reaching this level,” said Grossi. “Sixty per cent is almost weapons grade, commercial enrichment is 2, 3 [per cent].”

In an interview, he said it was Iran’s “sovereign right” to develop its programme, but added: “This is a degree that requires a vigilant eye.”

Iran has been ramping up its nuclear activity since May 2019 in response to Donald Trump’s decision to unilaterally pull the US out of the atomic accord Tehran signed with world powers and impose crippling sanctions on the Islamic republic.

Tehran announced last month it was enriching uranium at a purity level of 60 per cent — its highest-ever level — which far exceeds the 3.67 per cent purity agreed in the 2015 nuclear agreement. It has continued to increase its nuclear activity while holding talks with the accord’s remaining signatories — Germany, France, the UK, China and Russia — to thrash out an agreement that would lead to the US rejoining the deal.

President Joe Biden has said his administration would rejoin if Iran returns to full compliance. Tehran insists US sanctions must first be lifted. All sides have said the talks have been constructive. A fifth round began in Vienna on Tuesday.

Iran repeatedly denies it is seeking nuclear weapons. It had told the IAEA that its increased enrichment of uranium was for medical purposes and research, Grossi said. But he added: “We don’t seem to find much need for that at the current level of industrial, medical activity in Iran, but this is for a country to decide.”

Iran was complying with the accord before May 2019, but in the two years since, it has increased the number of operating centrifuges from the agreed limit of 5,060 to up to 7,000. It has also developed new centrifuges so it can produce more enriched uranium more rapidly. “Qualitatively, there’s been an important advancement,” Grossi said.

Iran’s stockpile of enriched uranium was now more than 10 times the 300kg limit agreed in the accord, he said.

Grossi said most measures taken by Tehran could be reversed relatively easily, but he added that the level of research and development that had taken place was an “issue”.

“You cannot put the genie back into the bottle — once you know how to do stuff, you know, and the only way to check this is through verification,” he said. “The Iranian programme has grown, become more sophisticated so the linear return to 2015 is no longer possible. What you can do is keep their activities below the parameters of 2015.”

Iran imposed restrictions on IAEA inspectors in February. But under a compromise agreement that was this week extended until six days after presidential elections in June, it has allowed some monitoring, while still limiting short-notice inspections. Grossi described the extension as a “bridge” but said the arrangement would ultimately be “unsustainable”.

“We are entering a phase where we have to take . . . one week at a time, and see how the other process [the Vienna negotiations] evolves,” Grossi said. “It’s obvious that with a programme with the degree of ambition, sophistication that Iran has, you need a very robust, very strong verification system . . . otherwise it becomes very fragile.”