President Donald Trump's respected defense secretary Jim Mattis added his heavyweight support to retaining the Iran nuclear deal Tuesday, as more hawkish voices pushed for tougher action against Tehran.

Under US law, Trump must write to Congress every 90 days to certify whether or not the White House believes Iran is keeping to its side of the bargain and whether or not the US should continue to waive sanctions.

Decertifying Iran's compliance would not necessarily signal the end of the 2015 accord, as long as the US Congress does not rush to reimpose the sanctions waived under the agreement. 

But as the latest deadline day approaches on October 15, Washington's warring foreign policy camps are deeply divided, and bombarding Trump's White House with wildly varying advice. 

Supporters of the 2015 deal argue that it remains the best way of halting Iran's progress towards building a nuclear bomb and that Washington must stand by its international commitments or lose face with friends and foes alike.

Critics argue the pact did nothing to rein in Iran's sponsorship of proxy armies destabilizing the Middle East nor to halt its aggressive ballistic missile drive -- and will leave it free to resume nuclear enrichment from 2025.

Trump himself signalled last month, in an address to the UN General Assembly, that he intends to "decertify" Iran's compliance with the Joint Comprehensive Plan of Action (JCPOA), under which the accord was implemented last year.

He branded the deal, which his predecessor Barack Obama's administration signed with five more world powers and Tehran, an "embarrassment" to the United States, and warned: "I don't think you've heard the last of it."

But his most senior foreign policy advisers have been more cautious.

Secretary of State Rex Tillerson's department has repeatedly agreed with UN nuclear inspectors that Iran is in "technical compliance" with the accord.

And on Tuesday, Trump's Pentagon chief Defense Secretary Jim Mattis told lawmakers the deal remains in the US national interest. 

"If we can confirm that Iran is living by the agreement, if we can determine that this is in our best interest, then surely we should stay with it," he told the Senate Armed Services Committee.

"I believe at this point in time, absent indication to the contrary, it is something that the president should consider staying with," he said.

Asked whether he believed the Iran deal was in the national interest, Mattis replied: "Yes, senator, I do."

Iran and the other signatories -- China, Russia, France, Britain and Germany -- defend the deal as a guarantee of the non-military purposes of Tehran's nuclear program, after Iran surrendered much of its enriched uranium and exposed its plants to inspection. 

- Banned missiles -

But Trump -- who told reporters last month he had reached a decision, but would not say what course of action he intends to take -- has appeared publicly to side with foreign policy hawks like his UN ambassador Nikki Haley.

Haley has argued that Iran's non-nuclear bad behavior, such as the regime's support for militia groups in Syria and Iraq and its banned missile program, justify non-certification.

And on Tuesday this banner was taken up by influential Republican senator Tom Cotton of Arkansas, who will be a key figure if Congress attempts to build a new sanctions package.

"I've long advocated for declining to certify the deal to Congress again for many reasons," Cotton was to say later Tuesday, according to an advance copy of a speech he was to deliver at the Council on Foreign Relations. 

"But President Trump has put it best himself: 'The Iran deal poses a direct national-security threat'," he was to say.

"The sensible course, then, is to decline to certify the deal and begin the work of strengthening it and counteracting Iranian aggression, with the threat of sanctions and military action if necessary."

Several senior US officials, as well as observers who oppose the deal, have said that if Trump does not certify Iran's compliance it would not necessarily mean a US withdrawal or the end of the pact.

In the event of non-certification, the law gives Congress 60 days to decide whether to reimpose on Iran sanctions lifted under the deal. 

Washington could use the time to pressure its European allies to reopen negotiations, and lawmakers like Cotton could lobby for non-nuclear related sanctions to target Iran's other sins.

Source: AFP