UN inspectors have found enriched uranium in environmental samples taken in Iran, and diplomats have speculated that Tehran has been purifying uranium without informing the International Atomic Energy Agency (IAEA).
Initial analysis showed enrichment levels possibly consistent with an attempt to make weapons-grade material and high enough to cause concern at the UN nucleur watchdog, according to anonymous diplomats.
"Mere presence of enriched uranium in the samples was not solid proof Iran had done the enrichment itself. Contamination was another possibility," said the diplomats.
The IAEA declined to confirm or deny the statements of the diplomats.
"The results of environmental sample analyses are being reviewed at the agency" a spokeswoman for the IAEA said. "Only the IAEA will be in a position to judge the significance of the analysis results," added the spokeswoman.
"At this point, we are still in the middle of a complex inspection process in Iran, in which we are investigating a number of unresolved issues," she added.
Iran insists it has no interest in building nuclear weapons, as Washington charges, but has resisted international calls for it to accept tougher inspections of its nuclear programme.
It was not clear at which of Iran's nuclear facilities the samples had been taken, nor was it clear whether the IAEA had informed the Iranian government of its initial findings.