An Iranian flag flutters in front of the International Atomic Energy Agency (IAEA) headquarters in Vienna, Austria, January 15, 2016. REUTERS/Leonhard Foeger

VIENNA (Reuters) - Iran has shipped 11 tonnes of heavy water abroad to bring its stock back under a limit set by its landmark nuclear deal with major powers, according to a diplomat citing a confidential U.N. nuclear watchdog report.

The shipment is a step toward resolving a dispute with Western powers including the United States that are keen to prevent Iran from testing the deal’s terms. The report substantiated an Iranian statement last month about a transfer to Oman but does not identify the destination, the diplomat said on Tuesday.

The International Atomic Energy Agency, which is policing the restrictions placed on Iran’s atomic activities under the July 2015 deal, said in a report last month that Iran’s stock of heavy water had for the second time exceeded a soft limit of 130 tonnes, and the IAEA expressed its concerns to Tehran.

“On 6 December the agency verified the quantity of 11 metric tonnes of the nuclear-grade heavy water at its destination outside Iran,” the diplomat quoted the five-paragraph report by the IAEA to member states as saying.

“This transfer of heavy water out of Iran brings Iran’s stock of heavy water to below 130 tonnes,” it said, adding that Iran had told the agency that the shipment left the country on Nov. 19.

Though the United States and its allies will see the shipment as a move in the right direction, it is not enough to satisfy them. Washington has underlined that the deal says excess heavy water must be delivered to a foreign buyer, and Iran has made clear Oman is not the final destination.

Heavy water is used as a moderator in nuclear power stations like Iran’s unfinished one at Arak that has had its core removed under the deal, which also lifted international sanctions against the Islamic Republic.

U.S. President-elect Donald Trump originally vowed to scrap the nuclear accord, describing it as “the worst deal ever negotiated” but later backed down, saying he would “police that contract so tough they (the Iranians) don’t have a chance”.

Iranian President Hassan Rouhani said on Tuesday he would not let Trump rip up the deal, warning of unspecified repercussions if Washington reneged on it.