Iran said on Tuesday it had sent 11 metric tonnes of heavy water to Oman as part of its obligations under last year's nuclear deal with world powers.
Ali Akbar Salehi, head of Iran's Atomic Energy Organisation, said the heavy water has been sold to an unnamed third country.
Heavy water is not itself radioactive, but is used in certain types of reactor which can produce plutonium that is used in a nuclear bomb.
"Eleven tonnes of heavy water has been sent to Oman and the other party has announced its readiness for the purchase," Salehi said, according to the IRIB news agency.
In a report this month, the International Atomic Energy Agency (IAEA) said Iran's stocks of heavy water had crept 100 kilos above the 130-tonne level set out in the nuclear deal with world powers that came into force in January.
Tehran denied last week that this breached the agreement.
"Iran has fulfilled its obligations on heavy water stockpiles," Atomic Energy Organisation spokesman Behrouz Kamalvandi said on Friday.
"We were required to put on the international market any excess over 130 tonnes, and so far we have sold 70 tonnes," he said.
The IAEA report said Iran has met its other obligations under the deal.
Tehran complains that it has not yet seen the expected benefits of the deal since sanctions were lifted in January.
Continuing US sanctions linked to Iran's ballistic missile tests and human rights record mean that international banks remain wary of doing business with the Islamic republic.