The US defense secretary, James Mattis, has backed the nuclear deal with Iran, saying it is in the interests of national security to maintain it, breaking with Donald Trump and potentially making it harder for the president to withdraw from the deal.



The timing and nature of Mattis’s remarks are particularly significant because Trump has threatened to withhold certification of the 2015 international agreement in a report to Congress due on 15 October . Under the relevant legislation, the administration has to certify whether Iran is in material breach of the agreement, or if the deal is not serving the national interest.

Mattis was asked at a hearing of the Senate armed services committee whether he believed it was currently in the US national security interest to remain in the agreement.

After a significant pause, the defense secretary replied: “Yes, senator, I do.”

At the same hearing, the chairman of the US joint chiefs of staff, General Joseph Dunford agreed that Iran was abiding by the deal, known as the Joint Comprehensive Plan of Action (JCPOA), which he said had delayed Iran’s acquisition of nuclear weapons. Last week, Dunford said the US should uphold the agreement, in the absence of a clear Iranian breach, or risk losing credibility when it came to signing future agreements.

Trump has repeatedly lambasted the JCPOA – one of the most important foreign policy legacies of his predecessor, Barack Obama – most recently at the UN general assembly last month.

“Frankly, that deal is an embarrassment to the United States and I don’t think you have heard the last of it, believe me,” he said, raising expectations that he would not endorse the agreeement.

Mattis is arguably the most powerful member of Trump’s cabinet and the president has been wary of contradicting him in public, something he has clearly no compunction about doing in the case of the secretary of state, Rex Tillerson.

Even if Mattis does not immediately sway Trump’s position on the JCPOA, his opinion is likely to carry weight with senators, who would be called on to decide the fate of the date, if the president did not certify it on 15 October.

Asked later in the committee hearing to explain his view on the deal, Mattis said. “The point I would make is if we can confirm that Iran is living by the agreement, if we can determine that this is in our best interests then clearly we should stay with it,” Mattis added. “I believe at this point in time absent indications to the contrary, it is something the president should consider staying with.”