"It gave the Iranian regime all the benefits upfront," he told CNN's Chris Cuomo on "New Day." "And then the incompleteness of the deal ... which really could give the regime cover to advance a nuclear program."
He said President Donald Trump is looking at the entire situation in Iran, not just its nuclear program, when deciding on the deal.
"We've taken a holistic look at this," McMaster said. "What's different about the President's approach is he didn't just look at the Iran deal -- he placed his decision on the Iran deal on broad context of how we protect American citizens, American interests, how we protect our allies and partners from Iran's broad range of destabilizing behavior."
McMaster pointed to Iran efforts to perpetuate violence across the Middle East, boost the Assad regim in Syria and support terrorist organizations.
He added: "I know what the decision is but when the President reveals that, when he talks about it, he'll place it context of the broader approach to Iran."
"Well, I have decided," Trump told reporters who pressed him for an answer before they were escorted out of his meeting with Mahmoud Abbas, the president of the Palestinian Authority.
Trump, however, declined to disclose his decision, teasing to a future