Trump, who on Tuesday called the 2015 international accord "an embarrassment," said he had made up his mind whether to keep the pact but declined to disclose his decision.
Trump must decide by Oct. 15 whether to certify that Iran is complying with the pact, a decision that could sink the deal. If he does not, the US Congress has 60 days to decide whether to reimpose sanctions waived under the accord.
A senior US official said Trump is leaning toward not certifying that Iran is complying with the pact and letting Congress effectively decide whether to kill the agreement.
The official said Trump could always change his mind before the deadline and noted he publicly and privately has fumed about the deal, feeling the United States was taken advantage of.
A source familiar with the US discussions said the Trump administration is also considering ways to leave the agreement intact, sanction Iran for its missile tests and support for extremist groups, and then seek to strengthen the pact.
Speaking at the UN General Assembly of world leaders, Iranian President Hassan Rouhani responded forcefully to Trump's pugnacious speech on Tuesday by saying Iran would not be pushed around by a relative newcomer to the world stage.
But he also said Iran desired to preserve its accord with six world powers under which Tehran agreed to restrict its nuclear program for at least a decade in return for the loosening of economic sanctions that crippled its economy.
"The Islamic Republic of Iran will not be the first country to violate the agreement," Rouhani said, adding that Iran would respond "decisively and resolutely" to a violation by any party.
"It will be a great pity if this agreement were to be destroyed by 'rogue' newcomers to the world of politics: the world will have lost a great opportunity," he said in a dig at Trump, who on Tuesday called Iran a "rogue" state.
Speaking later to reporters, Rouhani said he did not think Washington would leave the nuclear deal and said any country that abandoned the pact would isolate and embarrass itself.
"We don't think Trump will walk out of the deal despite (his) rhetoric and propaganda," Rouhani said.
"If American officials think that they can pressure Iran by walking out of the deal, they are making a big mistake," he added. "Either the nuclear deal remains as it is or it will collapse."
Trump, a businessman and former reality TV star whose first elected office is the presidency, told reporters, "I have decided," when asked if he had made up his mind after having criticized the accord in his own UN speech on Tuesday.