Iran denied on Friday that it had in any way breached its nuclear deal with world powers, insisting it was meeting its commitment to cap its stocks of controlled materials.
A report by the International Atomic Energy Agency last week showed that Iran's stocks of so-called heavy water had inched above the 130-tonne level set out in the agreement.
Heavy water is not itself radioactive but is used in certain types of nuclear reactor, which can in turn produce plutonium that can be used in an atomic bomb.
The July 2015 deal with world powers sets Iran's heavy water "needs" at 130 tonnes and states that any excess must be "made available for export".
Atomic Energy Organisation of Iran spokesman Behrouz Kamalvandi said arrangements were in hand to export the excess.
"Iran has fulfilled its obligations on heavy water stockpiles," state broadcaster IRIB quoted him as saying.
"We were required to put on the international market any excess over 130 tonnes and so far we have sold 70 tonnes," he said.
"Negotiations are under way with interested countries, in particular European," to sell the rest.
IAEA chief Yukiya Amano had chided Iran on Thursday for exceeding the agreed limit on its stockpiles for a second time.
"It is important that such situations should be avoided in future in order to maintain international confidence in the implementation," he said.
Washington has played down concerns about Iran's exceeding of the stockpile limit.
State Department spokesman Mark Toner said last week 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".
In all other respects, the IAEA found that Iran was continuing to abide by the agreement's terms.