Julian Borger, diplomatic editor guardian.co.uk, Thursday 5 November 2009 20.45 GMT
An Iranian long-range Shahab-3 missile being fired at an unspecified location. Photograph: Press TV/AFP/Getty Images
The UN's nuclear watchdog has asked Iran to explain evidence suggesting that Iranian scientists have experimented with an advanced nuclear warhead design, the Guardian has learned.
The very existence of the technology, known as a "two-point implosion" device, is officially secret in both the US and Britain, but according to previously unpublished documentation in a dossier compiled by the International Atomic Energy Agency (IAEA), Iranian scientists may have tested high-explosive components of the design. The development was today described by nuclear experts as "breathtaking" and has added urgency to the effort to find a diplomatic solution to the Iranian nuclear crisis.
The sophisticated technology, once mastered, allows for the production of smaller and simpler warheads than older models. It reduces the diameter of a warhead and makes it easier to put a nuclear warhead on a missile.
Documentation referring to experiments testing a two-point detonation design are part of the evidence of nuclear weaponisation gathered by the IAEA and presented to Iran for its response.
The dossier, titled "Possible Military Dimensions of Iran's Nuclear Program", is drawn in part from reports submitted to it by western intelligence agencies.
The agency has in the past treated such reports with scepticism, particularly after the Iraq war. But its director general, Mohamed ElBaradei, has said the evidence of Iranian weaponisation "appears to have been derived from multiple sources over different periods of time, appears to be generally consistent, and is sufficiently comprehensive and detailed that it needs to be addressed by Iran".
Extracts from the dossier have been published previously, but it was not previously known that it included documentation on such an advanced warhead. "It is breathtaking that Iran could be working on this sort of material," said a European government adviser on nuclear issues.
James Acton, a British nuclear weapons expert at the Carnegie Endowment for International Peace, said: "It's remarkable that, before perfecting step one, they are going straight to step four or five ... To start with more sophisticated designs speaks of level of technical ambition that is surprising."
Another western specialist with extensive knowledge of the Iranian programme said: "It raises the question of who supplied this to them. Did AQ Khan [a Pakistani scientist who confessed in 2004 to running a nuclear smuggling ring] have access to this, or is it another player?"