Diplomats say Iran has begun to export excess quantities of heavy water, a material that could be used in the process to make atomic arms, as it moves to end its violation of a landmark nuclear deal.

Heavy water is used to cool reactors that can produce substantial amounts of plutonium, which can be used for the core of nuclear warheads. A recent U.N. report said that Tehran had slightly more heavy water in storage than the 130 metric tons called for by the agreement between it and six world powers.

Iran has recently pledged to ship out five tons of the material and the diplomats said it now had begun exports to Oman. They demanded anonymity because they weren't authorized to comment on what are confidential developments.