It's time for the United States to hold Iran accountable.

The U.N. nuclear watchdog said on Thursday that Iran was endangering support for a landmark deal with major powers by twice overstepping a "soft" limit on sensitive material.

The International Atomic Energy Agency (IAEA) said in a report last week that Iran had slightly exceeded the 130-tonne limit on its stock of heavy water for a second time since the deal was put in place in January, although it had not concealed the fact and had proposed remedial action.

The six other countries that signed the deal, including the United States, have been muted in their criticism. But there are 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.

According to the text of a speech to the IAEA Board of Governors, agency head Yukiya Amano said it was "important that such situations should be avoided in future in order to maintain international confidence" in the implementation of the agreement.

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 such as Iran's unfinished one at Arak, which had its core removed and made unusable under the deal.

The international 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".

Read more about the infraction here.