United Nations diplomats are concerned Iran could be secretly raising its nuclear enrichment threshold.
A report issued by the International Atomic Energy Agency revealed traces of increased enriched uranium levels at Iran's underground atomic site.
The nuclear agency said its experts have found particles enriched up to 27 percent at the Fordo enrichment plant in central Iran. Those levels are closer to what's needed for nuclear arms, but substantially lower than the 90-percent level needed to make such a weapon.
The report cited a May 9 letter from Iranian officials suggesting any enrichment at 27 percent was inadvertent.
Iranian officials said the particles were produced "above the target value" and could have been for "technical reasons beyond the operator's control."