"What the president, though, has done is he has set out a marker, a marker to Iranians and to our allies and partners that we have to fix fundamental flaws in this deal," McMaster told Fox News in response to a specific question on the issue.
The Iranian nuclear deal is a "weak deal" that is being "weakly" monitored, he said.
As such President Donald Trump has made clear that he will not permit this deal to provide cover for what is "a horrible regime" to develop a nuclear weapon, he said.
"We know from their behaviour, their behaviour broadly in the region, and their behaviour within the agreement where they have walked up to the line, they have crossed the line several times in terms of the restrictions, that this is not a trustworthy regime. So, much more comprehensive monitoring is in order," he said, adding, the US is seeking implementation of the deal.
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"It's just to implement the agreement by going to sites, to fully implement the inspections of sites, the monitoring of suspicious sites within Iran," he said.
"Their programmes can advance and then they can go to industrial scale enrichment of uranium within a very short period of time and then bridge into a weapon, and that is just an unacceptable risk in the world," he added.
He said Trump is not walking away from the deal just yet.
"So, if he sees some real change, if he sees the ability of the Congress within US law to address some of these problems associated with the deal. In our legislation, the domestic law about the deal was really flawed because it was really just about cost reporting to each other," he said.
Trump has laid out a strategy for dealing with Iran's destabilising and dangerous hate filled behaviour, its behaviour toward its own people and its behaviour towards the region, he said.
McMaster also said Trump will not allow North Korea to threaten the US with a nuclear weapon.
"Our president has been really clear about this. He is not going to permit this rogue regime, Kim Jong-un, to threaten the United States with a nuclear weapon," he said.
"It's actually the opposite. It's having the opposite effect," he said noting that the options available with the US including the military one are constantly under refinement.
"We have a broad range of new capabilities coming into our armed forces, thanks to the president's focus on modernising the armed forces, addressing what had been a bow wave of deferred military modernisation," he said, adding the military are refining, improving plans every day.
Responding to a question, he said Kim Jong-un doesn't understand how serious the US is about his behaviour and the behaviour of his regime.
"The president has been very clear on that and I think it's beneficial to the safety and security of not only the United States but our great allies in South Korea and Japan and the world," the national security advisor said.