The UN nuclear watchdog said in a confidential report today Iran had given it a document which diplomats said included partial instructions for making the core of a nuclear bomb.
The US ambassador to the International Atomic Energy Agency (IAEA) said the disclosure raised concerns about weaponisation but other diplomats and a US nuclear expert were more cautious, saying more investigation was needed into the issue.
"Iran's full transparency is indispensable and overdue," said IAEA chief Mohamed ElBaradei in a confidential report to the agency's board of governors.
The report, seen by Reuters, said that among other documents it had found one "on the casting and machining of enriched, natural and depleted uranium into hemispherical forms".
One European diplomat described it as a "cookbook" for the enriched uranium core of a nuclear weapon. But a US nuclear expert, David Albright of the Institute for Science and International security, said it was far from a step-by-step guide to producing a bomb core.
"Iran has gone from saying it got nothing on this subject to (saying it got) a little bit," he said. "But the question remains: did Iran get more than it admitted to?"
The Iranians told the IAEA the document had come to them unsolicited from people linked to the nuclear black market set up by disgraced Pakistani nuclear scientist Abdul Qadeer Khan.
Iran says it wants to use nuclear power only to generate electricity and has the legal right to do so. While Iran had been "more forthcoming" in providing access to documents and information in some areas, questions on the peaceful nature of its nuclear plans remained, the report said.
The IAEA board meets on Thursday to decide whether to send Iran to the Security Council for failing to allay suspicions it is hiding a nuclear arms programme behind a civilian one.
"This (document) opens new concerns about weaponisation that Iran has failed to address," the US ambassador to the IAEA, Gregory Schulte, said in a statement.
Britain said the IAEA should investigate the questions raised in its latest report. "They only serve to reinforce existing concerns about Iran's true intentions about its nuclear programme," a Foreign Office spokesman said.
The IAEA report asked Iran to provide information on dual-use equipment and allow visits to sites such as those at Lavizan. Washington says one site was used for sensitive nuclear work but was bulldozed before IAEA inspectors could visit it. Iran's chief nuclear negotiator Ali Larijani said earlier the IAEA should justify its request for access to Lavizan.