The head of the UN atomic watchdog chided Iran on Thursday after a second minor breach this year by Tehran of the 2015 nuclear deal with major powers.
A report by the International Atomic Energy Agency last week showed that Iran's stock of so-called heavy water had inched above the level agreed under the landmark accord.
Heavy water, a modified form of normal water, is used in certain types of nuclear reactors.
"Iran has since made preparations to transfer a quantity of heavy water out of the country," which will bring it below the ceiling, IAEA chief Yukiya Amano told the agency's board.
"It is important that such situations should be avoided in future in order to maintain international confidence in the implementation" of the deal, he said in Vienna.
The July 2015 nuclear deal between Iran and six major powers states that Iran's "needs" are estimated to be around 130 metric tonnes of heavy water.
US State Department spokesman Mark Toner said that it was "important to note that Iran made no effort to hide this" and that he was "not sure whether that constitutes a formal violation".
Otherwise the IAEA's quarterly report, the fourth since the nuclear deal entered into force, confirmed that Iran continues to abide by the deal.
The number of uranium centrifuges in operation and Iran's uranium stockpile -- seen as much bigger areas of concern than heavy water -- were below agreed limits.
US president-elect Donald Trump during his campaign labelled the deal, which saw painful economic sanctions on Iran lifted, a "disaster" and threatened to tear it up.