Trump was responding to a question on whether he had made up his mind to exit the Iranian nuclear deal struck in 2015.
"I have decided," he repeated three times.
Sitting in a bilateral meeting with President Mahmoud Abbas of Palestinian Authority, Trump smiled when pressed on what the decision was.
"I'll let you know what the decision is," he said.
More From This Section
In their brief remarks to the press, Trump and Abbas expressed hope that they would be able to bring a lasting and peaceful resolution of the Israel-Palestine conflict.
"We are fighting very hard, we are trying very hard. If we can do it, it would be a great great legacy for everybody. So let's see what happens. We're looking seriously at peace and maybe ultimately peace in the whole of the Middle East. I think we have a pretty good shot, maybe the best shot ever," he said.
The Palestinian President said the meeting showed Trump's seriousness to "achieve the deal of the century" during this year or in the coming months.
Abbas said he was very certain that Trump was determined to bring peace in the Middle East.
"This gives us the assurance and the confidence that we are on the verge of real peace" between the Palestinian and the Israelis, he said.
"We can coexist peacefully together," he added.
"We're working together on many problems, and some things that aren't problems that are very, very good. But we're going to make some of the bad ones turn out good," Trump said.
King Abdullah said terrorism was a scourge around the world.
"But I think Jordan will always stand beside you and your country. And we will overcome," he said as they met on the sidelines of the UN General Assembly session in New York.
Trump's comment that he has made a decision on Iranian nuclear deal comes a day after he blasted Iran for its alleged support to terrorists.
"The longest suffering victims of Iran's leaders are, in fact, its own people. Rather than use its resources to improve Iranian lives, its oil profits go to fund Hezbollah and other terrorists that kill innocent Muslims and attack their peaceful Arab and Israeli neighbours," he said.
The nuclear agreement deal struck in July 2015 between the Obama administration, Iran, China, Russia, France, Germany and the UK requires that Iran severely curb its nuclear activities in return for global sanctions relief.
Under US law, the State Department must notify Congress every 90 days of Iran's compliance with the nuclear deal.