Evidence found in an underground bunker in Iran could signal the country’s having moved one step closer toward the uranium threshold needed to make nuclear arms, International Atomic Energy Agency diplomats said today.

IAEA inspectors found traces of uranium enriched up to 27 percent at Iran’s Fordo enrichment plant, the Associated Press reported.

While still well below the 90-percent needed for a nuclear weapon’s fissile core, the figure is Iran’s highest-known enrichment grade yet. It also is well above the Islamic Republic’s main stockpile, which can only be used for fuel at around 3.5 percent.

The diplomats stressed this did not necessarily mean that Iran was pushing ahead toward weapons-grade level material. One possible explanation, they explained, was that the centrifuges that produce enriched uranium initially over-enriched at the start of the process 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. IAEA media officials said the agency had no comment.

Iran started enriching to 20 percent last year, mostly at Fordo, saying it needed the material to fuel a research reactor and for medical purposes.

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