Iran sent a major shipment of low-enriched uranium materials to Russia on Monday, a key step in Tehran's implementation of this year's historic nuclear accord with world powers.
The United States hailed the move, which Secretary of State John Kerry said marked "significant progress" in Tehran's fulfilment of a deal to stop it developing nuclear weapons.
The Russian foreign ministry confirmed the report after Ali Akbar Salehi, head of Iran's Atomic Energy Organization, told the ISNA news agency: "The fuel exchange process has taken place."
According to ISNA's report, Iran sent 8.5 tonnes of low-enriched nuclear material to Russia and received about 140 tonnes of natural uranium in return.
Mark Toner, the US State Department spokesman, described the cargo as a 12.5 tonne "combination of forms of low-enriched uranium materials" including 5 and 20 percent enriched uranium, scrap metal, and unfinished fuel plates.
"So that actually constitutes, I think, almost all of Iran's current stockpile of enriched uranium," Toner said.
Under the deal struck in July in Vienna between Iran and the P5+1 group of world powers, Tehran agreed to cut its low-enriched uranium (LEU) stockpile to less than 300kg.
This would mean that it would not have enough fuel on hand to rapidly enrich enough to the levels needed to build a nuclear weapon.
Kerry said the move was "a significant step towards Iran meeting its commitment to have no more than 300kg of low-enriched uranium by Implementation Day".
"Implementation Day", which could come as early as next month, will arrive after the UN nuclear watchdog, the International Atomic Energy Agency, rules that Tehran has complied with its obligations.
The IAEA had no immediate comment on Monday's shipment, and Toner said the US would wait until the UN agency makes a ruling to say formally whether Washington now accepts that Iran has less than 300kg.
After Iran's compliance has been verified, the US and its allies will begin to dismantle the sanctions imposed on Tehran in response to its nuclear programme.