With just over a week before US President Donald Trump must again publicly state whether he intends to certify Iran's compliance to the 2015 nuclear deal, US Secretary of Defense Jim Mattis warned on Tuesday that it was in the country's national interest to remain a part of the accord unless it were proven that Tehran was not adhering to the to the agreement.

Speaking in a Senate hearing, Mattis said: "if we can confirm that Iran is living by the agreement, if we can determine that this is in our best interest, then clearly we should stay with it. I believe, absent indications to the contrary, it is something that the president should consider staying with."

Asked whether he thought remaining in the accord was in US national security interest, Mattis replied, "Yes, senator, I do."

The accord, reached between Iran and six world powers in 2015, restricts Iran's uranium enrichment and stockpiles in exchange for easing of international sanctions. 

Trump, however, has repeatedly decried the deal, having described it as the "worst and most one-sided transactions" the United States had ever signed. While campaigning for the presidency in 2016, he said he would rip up the agreement.

Read more: What are Donald Trump's objections to the Iran nuclear deal?

Violation of 'spirit of the deal'

Under a US law known as the Iran Nuclear Agreement Review Act, the president must write to Congress every 90 days to certify whether the White House believes Iran is abiding by the deal. The next deadline for Trump to issue a written decision is on October 15.

Along with Mattis, both the US State Department and the International Atomic Energy Agency confirm that Iran has abided by the agreement, and nuclear non-proliferation experts and other international powers that brokered the deal are pressing the White House to stay in.

White House officials said Tehran has been in "technical compliance" with the agreement, but they have also accused Iran of violating the "spirit of the accord" by conducting non-nuclear ballistic missile tests and for supporting Syrian President Bashar al-Assad's regime in the Syrian civil war.

Read more: Iran's Hassan Rouhani vows to boost missile defenses despite US criticism

In the event of 'decertification'

After twice reluctantly certifying the accord, Trump has recently appeared to side with the foreign policy hawks in his administration. During his UN General Assembly speech last month, the president signaled that he intended to "decertify" Iran's compliance, although he has refused to share his view ahead of the October 15 deadline.

Should Trump refuse to certify Iran's compliance, it would not necessarily mean an immediate US withdrawal from the pact, according to US political sources and observers. Instead, the law would allow Congress to decide within 60 days to reimpose some of the sanctions against Iran that were lifted under the deal.

The Trump administration would then likely try to push the accord's other co-signatories — Britain, China, France, Germany, Russia — to reopen negotiations with Tehran and impose non-nuclear related sanctions that would target Iran's missile tests and role in Syria. None of those countries has expressed an interest in reopening negotiations.

Should that fail and the Congress, where Republicans hold a majority in both houses, decides to once again impose sanctions Iran, the US would then be effectively withdrawn from the accord.

dm/sms (AP, Reuters, dpa)