Rex Tillerson said the American president was asked by U.K. Prime Minister Theresa May during their bilateral meeting on the sidelines of the ongoing UN General Assembly, but "he said no".
Secretary Tillerson looked surprised when a reporter informed him that the president had already told the press that he had indeed arrived at a final decision, signaling disconnect between the head of government and the top diplomat.
"I didn't know he was going to say today he made a decision," Tillerson said. "I knew he had, but I didn’t know he was going to say he had."
Tillerson held the press briefing following a meeting with the parties to the deal, known officially as the Joint Comprehensive Plan of Action, or JCPOA.
The meeting marked the highest-level U.S.-Iran contact between Tillerson and Foreign Minister Javad Zarif since Trump entered the Oval Office.
After the meeting, European Union foreign policy chief Federica Mogherini said all parties, including Tillerson, agreed that the deal "is working and is delivering for its purpose".
Mogherini appeared to object to President Trump's public criticism of the deal, saying: "The international community cannot afford to dismantle an agreement that is working."
Secretary Tillerson conceded that reports from the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), "continue to confirm that Iran is in technical compliance with the agreement".
He said however that in the broader context, "the aspirations" of the deal had not been met.
Tillerson said that "sunset clauses" that allow Tehran to resume its nuclear program after a certain period of time were a primary shortcoming of the agreement.
"One can almost set the countdown clock to Iran resuming its nuclear activities," he said.
During his first UN address as president, Trump unleashed a stern rebuke of Iran, calling it a "rogue regime" and the deal "an embarrassment to the United States".
Iranian President Hassan Rouhani retaliated during his address at the general assembly on Wednesday by calling Trump a "rogue newcomer to the world of politics", without naming him.
By violating its international commitments, the new US administration only destroys its own credibility and undermines international confidence in negotiating with it, or accepting its word or promise," Rouhani said.