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known as a "two-point implosion" device, is officially secret in both the US and Britain, but according to previously unpublished documentation in a dossier compiled by the International Atomic Energy Agency (IAEA), Iranian scientists may have tested high-explosive components of the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.

To read full London Guardian story — Go Here Now.