* Trump says has made up his mind on Iran deal, but won't say how
* U.S. official sees Trump effectively handing Congress decision
* U.S., Iranian, major power diplomats begin meeting
* Iran says it will not be first to violate agreement (Adds Iran, major powers meeting began, comments on U.S. strategy)
UNITED NATIONS, Sept 20 (Reuters) - Iran vowed on Wednesday not to be the first nation to violate the Iran nuclear deal and said it did not expect the United States to abandon it despite President Donald Trump's fierce criticism.
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 U.S. Congress has 60 days to decide whether to reimpose sanctions waived under the accord.
A senior U.S. 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 U.S. 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 U.N. 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 U.N. speech on Tuesday.
But he declined to say what he decided.
U.S. officials have sent mixed signals about the nuclear agreement Iran hammered out with six major powers - Britain, China, France, Germany, Russia and the United States.
On Wednesday, U.S. Ambassador to the United Nations Nikki Haley said Trump's speech signaled "that he's not happy with the deal" but not a decision to abandon the accord.
On Tuesday evening, however, U.S. Secretary of State Rex Tillerson told Fox News that "if we're going to stick with the Iran deal there has to be changes made to it."
The Republican president hinted on Tuesday that he may not recertify the pact, negotiated by his Democratic predecessor, Barack Obama. "I don't think you've heard the last of it," he said.
'MORE PAINFUL RESPONSES'
Clues to the U.S. stance might emerge on Wednesday when the seven parties to the agreement met at the United Nations, marking the first time Tillerson and Iranian Foreign Minister Mohammad Javad Zarif are to meet in the same room.
The head of Irans Revolutionary Guards earlier said the United States should experience "painful responses" following Trump's harsh criticism.
The prospect of Washington reneging on the agreement has worried some U.S. partners that helped negotiate it, especially as the world grapples with North Koreas nuclear and ballistic missile development.
French President Emmanuel Macron said it would be a mistake to pull out of the pact. "We have to keep the 2015 agreement because it was a good one," he told reporters.
Russia is also concerned by Trump questioning the deal, Foreign Minister Sergei Lavrov told reporters in comments published by his ministry on Wednesday. (Reporting by Yara Bayoumy, Parisa Hafezi, John Irish, Michelle Nichols, Jeff Mason and Arshad Mohammed at the United Nations, Susan Heavey in Washington, Andrew Osborn in Moscow and Babak Dehghanpisheh in Beirut; Writing by Arshad Mohammed; Editing by Grant McCool and James Dalgleish)