Although it is still substantially below the 90-per cent level needed to make the fissile core of nuclear arms, it is above Iran's highest-known enrichment grade, which is close to 20 per cent. It can be turned into weapons-grade material much more quickly than the country's main stockpile, which can only be used for fuel at around 3.5 per cent.
The diplomats, who sought anonymity because their information is privileged, said the evidence did not necessarily mean that Iran was covertly raising its enrichment threshold toward weapons-grade level. They said one likely explanation was that the centrifuges that produce enriched uranium were over-enriched at the start as technicians adjusted their output. Calls to Ali Asghar Soltanieh, Iran's chief delegate to the IAEA, were rejected and the switchboard operator at the Iranian mission said he was not available.
Iran is under several rounds of UN sanctions for its failure to disclose information on its controversial nuclear programme. While Tehran said it was enriching uranium for civilian purpose, the US and other nations fear that it was doing so to make nuclear weapons.