VIENNA (Reuters) - The United States and Iran on Thursday clashed openly at the U.N. atomic watchdog for the first time since they signed a landmark nuclear deal last year, differing over Tehran’s repeated testing of one of the deal’s less strictly defined limits.

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

The International Atomic Energy Agency (IAEA), which is policing the deal, said Iran’s overstepping of the limit on its stock of a sensitive material for the second time this year risked undermining countries’ support for the agreement.

The victory of Donald Trump - a vocal critic of the deal - in the U.S. presidential election also raised the question of whether his country would continue to support the accord, which restricts Iran’s nuclear activities in exchange for the lifting of sanctions against the Islamic Republic.

“Iran must strictly adhere to all commitments and technical measures for their duration,” U.S. ambassador to the IAEA Laura Holgate said in a statement to the agency’s quarterly Board of Governors meeting.

The dispute centers on the part of the deal between Tehran and six major powers that limits Iran’s stock of heavy water, a material used as a moderator in reactors like the unfinished one it has at Arak that has been put out of use.

In contrast to strict limits elsewhere in the deal on materials including enriched uranium, the text says Iran should not have more heavy water than it needs, adding that those needs are estimated to be 130 tonnes. Western countries see it as a hard limit, and Iran argues it is not.

“We note with concern Iran’s accumulation of heavy water in excess of the limit set forth in the JCPOA of 130 metric tonnes,” Holgate said, using the abbreviation for the deal’s full name, the Joint Comprehensive Plan of Action.

The IAEA said Iran was preparing to ship some heavy water out of the country to come back under the 130-tonne limit, but Holgate said Iran would not be in compliance until it had been delivered to a foreign buyer as the deal requires.

“Simply notifying states that this heavy water is for sale without removing it from Iran does not fulfill this JCPOA commitment,” she said.

Iran said the issue was not that clear-cut.

“Where is (the) limit?” Iran’s envoy to the IAEA, Reza Najafi, told reporters on the sidelines of the board meeting, adding that the country was preparing to export more than the 5 tonnes of heavy water it originally informed the IAEA of.

“The JCPOA is very clear,” he added. “It says that the needs of Iran are estimated (to be) 130 tonnes. Who is the native English speaker to tell me what estimated means?”