"The point I would make is that 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," Mattis told the Senate Armed Services Committee on Tuesday. "I believe at this point in time, absent indications to the contrary, it is something that the president should consider staying with.”

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Iran is "not in a material breach of the agreement," said General Joseph Dunford, chairman of the Joint Chiefs of Staff. “I believe that [the agreement] has delayed Iran's nuclear program."

Under the deal negotiated in July 2015, Iran agreed to reduce the number of its uranium enrichment centrifuges by two-thirds, cap its enrichment below the level needed for weapons-grade material, reduce its enriched uranium stockpile by 98 percent from around 10,000kg for 15 years, and allow international inspections.

In exchange, much of the international sanctions against Tehran have been lifted. Some US sanctions, however, still remain in effect.

On the campaign trail, Trump vowed to destroy the deal negotiated by the Obama administration, calling it "disastrous" while speaking before the American-Israeli lobby in Washington, DC.

In February, he formally put Iran on notice for firing a ballistic missile, once again calling the deal "terrible."

Senior US officials have repeatedly accused Iran of violating the "spirit" of the nuclear deal by testing ballistic missiles. Tehran says its missile program is entirely unrelated.

Iranian President Hassan Rouhani said in September that Washington would pay a "high cost" if it pulled out of the nuclear deal.

The International Atomic Energy Agency (IAEA) said in February that Tehran had been found in full compliance with the nuclear deal.

Citing data as of February 18, the watchdog's report put Iran's low-enriched uranium stockpile at 101.7 kg - only half of the permitted amount of 202.8 kg.