And at a closed-door meeting with diplomats, the International Atomic Energy Agency's chief for inspections, Olli Heinonen, revealed that the agency had gathered intelligence from around 10 countries suggesting Iran was engaged in weaponisation studies in the past, the diplomat said.
Tehran has repeatedly dismissed the intelligence as "fabricated," and the allegations that it was seeking to build a bomb as "baseless".
At a briefing to prepare IAEA board members for a meeting of the full board next week, Heinonen talked about the so-called uranium metal document, the diplomat said on condition of anonymity.
The 15-page document describes the process of machining uranium metal into two hemispheres of the kind used in nuclear warheads.
"And the term he used for this document was 'alarming'. He essentially said there was no reason why a country would need to possess such a document unless they wanted to produce uranium hemispheres for a nuclear weapon," the diplomat said.
Iran has told the IAEA that it received the document back in 1987 along with design information for the so-called P1 centrifuges used to enrich uranium.
Tehran insists it did not request the uranium metal document.
But the IAEA said in its latest report that it needed to understand the precise role of the document so that it could determine the true nature of Iran's disputed nuclear programme.
When contacted by AFP, Heinonen declined to comment on what he had said at the briefing, which he described as an "informal technical meeting".
But another diplomat close to the Vienna-based IAEA confirmed that Heinonen, who is the agency's deputy director general, had used the term "alarming" in the context of the uranium metal document.
In a sternly-worded report released on Monday, the IAEA expressed "serious concern" that Iran is hiding information about the alleged weaponisation studies, as well as defying UN demands to suspend uranium enrichment.
Experts and observers detected a tougher tone in the language of the report, suggesting that the IAEA was becoming frustrated by Iran's persistent stonewalling of its investigations.
"It's one of the toughest I've seen," a western diplomat said Thursday.
The report, a copy of which was obtained by AFP, said the IAEA was "of the view that Iran may have additional information" regarding intelligence that suggested it may have looked into high explosives of the sort used in implosion-type nuclear bombs, and explored modifications to missiles consistent with making them capable of delivering a nuclear weapon.
Iran's ambassador to the IAEA, Ali Asghar Soltanieh, speaking to reporters after the briefing Thursday, again dismissed the intelligence as "lousy" and "fake" and accused the United States was trying to influence the IAEA inspectors for its own political ends.
One of the Tehran's main arguments against the authenticity of the intelligence is that none of the documentation had any official stamp marking it as confidential or top secret.
That and numerous other discrepancies were proof that the intelligence was "forged and fabricated," Soltanieh said.
Western countries such as the United States remain unconvinced by such statements, however, and insist the onus is on Tehran to actively disprove the allegations rather than simply dismiss them as untrue.
"As today's briefing showed us, there are strong reasons to suspect that Iran was working covertly and deceitfully, at least until recently, to build a bomb," said the US ambassador to the IAEA, Gregory Schulte.
"Iran has refused to explain or even acknowledge past work on weaponisation," Schulte said.
"This is particularly troubling when combined with Iran's determined effort to master the technology to enrich uranium. Uranium enrichment is not necessary for Iran's civil programme but it is necessary to produce the fissile material that could be weaponised into a bomb," Schulte said.
