Mattis backs Iran deal

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.

The Pentagon chief also tried to clear up doubts about the US administration's North Korea strategy, backing Secretary of State Rex Tillerson's effort to find a diplomatic solution to the nuclear standoff.

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.

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.

On Tuesday, Trump's Pentagon chief told lawmakers the deal remains in the US national interest.

Mattis was also clear about his stand on how to solve North Korea crisis.

Mattis put his support behind Tillerson two days after Trump appeared to undermine his top diplomat by saying was "wasting his time" by maintaining contacts with Kim Jong-Un's regime.

"The international community... is focused on the destabilizing threat posed by North Korea and Kim Jong-Un's relentless pursuit of nuclear and ballistic missile capabilities," he said.

"The Defense Department supports fully Secretary Tillerson's efforts to find a diplomatic solution but remains focused on defense of the United States and our allies."