In a report to IAEA governors on Friday, Mr Grossi said his inspectors had confirmed Iran was feeding more partially enriched uranium into the cascades of two centrifuges at its Fordow nuclear plant south of Tehran.
“The facility’s updated design information showed that the effect of this change would be to significantly increase the rate of production of UF6 (uranium) enriched up to 60%,” the report said. It assessed the facility would produce 34kg (75lb) of 60% uranium per month compared previously with 4.7kg.
The IAEA had demanded further “safeguard measures” at Fordow “as a matter of urgency to enable the agency to provide timely and technically credible assurances that the facility is not being misused to produce uranium of an enrichment level higher than that declared by Iran, and that there is no diversion of declared nuclear material”.
Iran denies having a military nuclear programme. But Mr Grossi told the BBC its nuclear energy facilities had increased over the last decade.
“They have a nuclear programme that has grown, has spawned in every possible direction.
“The Iran of 2015 has nothing to do with Iran of 2025. Iran is starting production of 60% [uranium] at a much higher level of production, which means they will have the amounts necessary – if they so choose – to have a nuclear device in a much faster way. So we see an escalation in this regard, which is very worrisome.”
On a visit to Tehran last month, Mr Grossi said he had been given an assurance by Iranian leaders that they would limit their production of 60% enriched uranium.
Iran’s decision to increase production comes after little progress was made in nuclear talks between European and Iranian officials last week.
Mr Grossi said there were groups in Iran that were “very vocal” calling for the country to “do its own thing” on nuclear weapons.
“In my conversations with the government, that doesn’t seem to be the path of choice, but they sometimes refer to this as something they might need to reconsider. I hope not. I have told them this would be a regrettable choice.”
Israel has not yet launched a full-scale attack on Iran’s nuclear facilities – but its ministers have openly discussed the possibility.
Asked about the consequences of any such Israeli attack, Mr Grossi said: “I don’t think this would go without an answer, militarily speaking, so I think we need to avoid this.
“One should not forget that a big part of the nuclear programme in Iran is underground and very well protected. So kinetic action against the programme would require a vast deployment of force.
“I just hope that we are not going to get there. I know the radiological consequences if you attack a nuclear facility.”
Mr Grossi also warned that the world’s nuclear non-proliferation regime was under stress, as established nuclear powers “seem to be relying more on nuclear weapons or modernising their arsenals”.
As a result, other nations were talking more about acquiring nuclear weapons.
“There are countries saying: well, why not us? If we see that we have a world… with new conflicts, the big [countries] are saying that perhaps they might use the nuclear weapons they have, maybe we should think about our own security.”
