But Mr Trump has been a frequent critic and on Thursday again called it "one of the worst deals I've seen".
His words came as the US imposed fresh sanctions on people accused of supporting Iranian cyber attacks.
"Certainly at a minimum the spirit of the deal is atrociously kept," Mr Trump told reporters on Air Force One.
In April, Mr Trump ordered a review into whether a suspension of sanctions on Iran, linked to the deal curbing its nuclear activities, was in the US national interest.
He is due to decide by 15 October whether Iran has breached the deal, and could withdraw his approval.
"You'll see what I'm going to be doing very shortly in October," he said.
"The Iran deal is one of the worst deals I've ever seen, certainly at a minimum the spirit of the deal is atrociously kept. The Iran deal is not a fair deal to this country. It's a deal that should not have ever been made...
"We are not going to stand what they are doing with our country. They've violated so many different elements and they've also violated the spirit of that deal."
Iran has always said it has the right to nuclear energy - and insists that its nuclear programme is for peaceful purposes only.
On Monday, the International Atomic Energy Agency said Iran was implementing the nuclear-related commitments from the 2015 deal.
But speaking in London, US Secretary of State Rex Tillerson cited the preface of the deal, which calls on Iran to contribute positively to regional security.
"In our view, Iran is clearly in default of these expectations... through their actions to prop up the Assad regime (in Syria), to engage in malicious activities in the region, including cyber activities, aggressively developing ballistic missiles.
"We have to consider the totality of Iran's activities and not let our view be defined solely by the nuclear agreement," he said.
Despite the criticism, the US has again extended some sanctions relief for Iran.
But state department spokeswoman Heather Nauert said: "Waiving some of those sanctions should not be seen as an indication of President Trump or his administration's position on the (Iran nuclear deal).
"Nor is the waiver giving the Iranian regime a pass on its broad range of malign behaviour."
At the same time, the US Treasury Department has announced sanctions on 11 individuals and entities, accused of supporting Iranian cyber attacks and other aggressive acts.
The move freezes any assets they may hold in the US and prohibits Americans from doing business with them.
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