VIENNA (Reuters) – Iran must stop repeatedly overstepping a limit on its stock of a sensitive material set by its landmark deal with major powers, the U.N. nuclear watchdog said on Thursday.

The International Atomic Energy Agency (IAEA), which is policing the deal, said in a report last week that Iran had slightly exceeded the 130-tonne soft limit on its stock of heavy water for a second time since the deal was put in place in January.

Officials from the six other countries that signed the deal, including the United States, have expressed frustration over the breach and said the limit should be seen as firm.

Iran’s overstepping of the 130-tonne threshold also raises questions about how U.S. President-elect Donald Trump – who has strongly criticized the deal and said he will “police that contract so tough they (the Iranians) don’t have a chance” – would handle any similar case once he takes office.

“It is important that such situations should be avoided in future in order to maintain international confidence in the implementation of the JCPOA,” IAEA chief Yukiya Amano said in the text of a speech to his agency’s Board of Governors, using the acronym for the deal’s full name, the Joint Comprehensive Plan of Action.

Last week’s report said Amano had expressed “concerns” to Iran over its stock of heavy water, a material used as a moderator in reactors like Iran’s unfinished one at Arak, which had its core removed and made unusable under the deal.

The agreement places restrictions on Iran’s atomic activities – monitored by the IAEA – in exchange for the lifting of international sanctions against the Islamic Republic.

Rather than setting a strict limit on heavy water as it does for enriched uranium, the deal estimates Iran’s needs to be 130 tonnes and says any amount beyond its needs “will be made available for export to the international market”.

“Iran has … made preparations to transfer a quantity of heavy water out of the country,” Amano said, without saying when the transfer would take place. “Once it has been transferred, Iran’s stock of heavy water will be below 130 metric tonnes.”

(Reporting by Francois Murphy; Editing by Gareth Jones)