But Mr Abbasi-Davani said research went further and had a military aspect. 

“Although our stance on nuclear weapons based on the supreme leader’s explicit fatwa regarding nuclear weapons being forbidden is quite clear, Fakhrizadeh created this system and his concern wasn’t just the defence of our own country,” he said.

The interview with the Islamic Republic News Agency was billed as an expose of lesser-known aspects of Fakhrizadeh’s life and came ahead of Iran resuming negotiations with world powers over returning to the nuclear deal. 

The Vienna talks are the first on Iran’s nuclear programme since Iran elected a new hardline president Ebrahim Raisi in August, following six rounds of fruitless talks between April and June. 

Iran had refused to meet directly with the United States since former US president Donald Trump abandoned the nuclear deal in 2018, arguing it did not go far enough in limiting Tehran’s nuclear programme. 

Since then Iran has progressively walked back from its commitments under the deal, enriching uranium beyond the 3.67 percent limit set by the 2015 agreement. 

This year Iran has enriched uranium to 60 per cent, much closer to the 90 per cent needed for a nuclear weapon. Iran says it is enriching uranium only for civilian uses, though experts say there are few legitimate peaceful applications for such high levels of enrichment.

Hopes of a breakthrough at the latest talks appear slim, with Tehran adopting a strict stance and requesting guarantees that the US would not leave the deal again.  

“Our demands are clear. Other parties and especially Americans should decide whether they want this deal to be revived or not. They abandoned the pact, so they should return to it and lift all sanctions,” an Iranian official close to the talks told Reuters.

Iran is demanding the lifting of all US and European Union sanctions imposed since 2017, including those unrelated to its nuclear programme.

“The United States still fails to properly understand the fact that there is no way to return to the deal without a verifiable and effective lifting of all sanctions,” Iranian Foreign Minister Hossein Amirabdollahian said in a statement shortly after the talks resumed.

“The return of the U.S. to the nuclear deal would be meaningless without guarantees to prevent the recurrence of the bitter experience of the past,” he said. “This opportunity is not a window that can remain open forever.”

Western diplomats have said these demands will be unrealistic for the US and other signatory powers – Britain, China, France, Germany and Russia.

Foreign Secretary Liz Truss called the negotiations “the last opportunity for the Iranians to come to the table” after a meeting with Israeli Foreign Minister Yair Lapid.

“We want those talks to work,” Ms Truss said. “But if they don’t work, all options are on the table.”

Experts say that Iran could have enough enriched uranium for a bomb within six weeks but that it could take up to two years for it to develop it into a functioning a warhead.

“Enrichment to 60 percent could be around 99 percent of the effort to reach weapons-grade, which underscores the gravity of the situation,” nuclear nonproliferation researcher Andrea Stricker told AFP.