U.S. Secretary of State John Kerry, left, meets with Iranian Foreign Minister Mohammad Javad Zarif, right, in Vienna, Austria, Saturday, Jan. 16, 2016, on what was expected to be “implementation day,” the day the IAEA verifies that Iran has met all conditions under the nuclear deal.

Iran briefly exceeded a limit set by a deal with major powers under which sanctions against it were lifted, the U.N. nuclear watchdog said on Friday, but Tehran then came back within the permitted bounds.

Under its July deal with the United States, Russia, China, France, Britain and Germany, Iran is allowed to have 130 tons of heavy water, a moderator in reactors like the one it has disabled at Arak and a chemical it produces itself.

“On 17 February, the agency verified that Iran’s stock of heavy water had reached 130.9 metric tons,” the International Atomic Energy Agency (IAEA), which polices the deal, said in a regular report on Iran’s nuclear program sent to its member states.

By Wednesday, however, 20 tons of heavy water had been shipped out of the country, bringing the stock back under the threshold of 130 tons, apparently in keeping with a soft limit under the terms of the July 14 deal, which is formally called the Joint Comprehensive Plan of Action (JCPOA).

“All excess heavy water which is beyond Iran’s needs will be made available for export to the international market,” one of the annexes in the deal stipulates, adding: “Iran’s needs are estimated to be 130 metric tons.” In Washington, a U.S. official who spoke on condition of anonymity played down the incident.

“Iran briefly exceeded its 130 metric ton heavy water stockpile limit under the JCPOA by less than one ton. The IAEA has now verified that Iran has shipped out 20 metric tons and is back well under this limit,” said the U.S. official.

“Iran made no effort to hide anything it was doing from the IAEA. Because of the enhanced monitoring and verification provisions in the JCPOA, the IAEA immediately became aware of this issue and raised it with Iran, and Iran fixed it,” he said.

“It is not surprising that there are challenges for Iran in ensuring it is meeting all of the many nuclear commitments in the early stages of implementation of the JCPOA, but again, this issue has been resolved,” the U.S. official added.

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