By Daniel Dombey
In 2002, an exiled Iranian opposition group that the US designated as a terrorist organisation announced that the Islamic Republic was engaged in clandestine nuclear activities at specific sites.
The claim, subsequently proved to be true, has been at the heart of suspicions about Iran's nuclear programme ever since. In the most important site to come to light, an underground facility in Natanz, Iran had started working towards small-scale uranium enrichment - a process that at high levels can produce weapons-grade material.
Tehran, which subsequently allowed international inspectors into the country, has always insisted its nuclear programme is purely peaceful. The International Atomic Energy Agency, the United Nations nuclear watchdog, has been careful to say that it has not found any proof that the country has been operating a weapons programme.
But the IAEA has grown increasingly frustrated by what it sees as Iran's foot-dragging over its access to places, people and documents - in spite of a spurt of co-operation over the past week - and its language has grown harsher.
In a note circulated for today's IAEA board meeting, which is set to report Iran to the UN Security Council, the agency sets out some of the most worrying information to have emerged since the Natanz site was revealed.
The note refers to a 15-page document, which the Iranians showed to the IAEA last year, containing instructions for "the casting of enriched and depleted uranium metal into hemispheres, related to the fabrication of nuclear weapons".
Iranian officials argue that they had not sought such information but that it had been provided by Tehran's supplier, the black-market nuclear network operated by Abdul Qadeer Khan, the father of Pakistan's nuclear programme.
The IAEA note also cites allegations - thought to come from a western intelligence agency and formally passed on to the IAEA relatively recently - on uranium feedstocks, "tests related to high explosives and the design of a missile re-entry vehicle, all of which could have a military nuclear dimension and which appear to have administrative interconnections". These Iran calls baseless.
The IAEA note adds that enrichment-related work has been carried out at military sites.
Mark Fitzpatrick, a senior fellow at the International Institute for Strategic Studies in London, says Iran is at least five years away from developing a nuclear bomb if it wants to - assuming that no other nuclear facilities have been hidden away.
"The five years are based on the assumption that they go for broke and are ignoring international reaction," he says. "And right now, that looks like what they are doing. The clock is ticking."
