The UN nuclear agency has found traces of uranium at Iran's underground atomic site enriched to higher than previous levels and closer to what is needed for nuclear weapons, diplomats say.
The finding by the International Atomic Energy Agency does not necessarily mean that Iran is secretly raising its enrichment threshold.
The diplomats say the traces could be left during start-up of enriching centrifuges until the desired level is reached. That would be a technical glitch only.
But they say the agency is investigating the find because the higher the level of enrichment, the easier it is to turn uranium into nuclear warhead material.
The diplomats, who asked not to be named, all follow the IAEA's monitoring of Iran's nuclear programme.
The West suspects Iran is trying to develop nuclear weapons while Tehran insists its programme is for nuclear energy and research.
Sanctions have been imposed on Iran and Israel has warned it may take military action to halt the nuclear programme.
Negotiations between Iran and world powers over its nuclear programme ended are to continue in Moscow next month after two days of talks in Baghdad ended yesterday.