* 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 meet
* Iran says it will not be first to violate agreement (Updates with end of meeting, Mogherini quotes)
UNITED NATIONS, Sept 20 (Reuters) - Iran said on Wednesday it did not expect the United States to abandon the Iranian nuclear deal as U.S. officials sent mixed signals on what they plan to do about the international accord.
A collapse of the 2015 deal, which U.S. President Donald Trump has called "an embarrassment" but which is supported by the other major powers that negotiated it with Iran, could upend relations in the Middle East and trigger a regional arms race.
Iranian President Hassan Rouhani vowed that his country would not be the first to violate the agreement under which Tehran agreed to restrict its nuclear program in return for the loosening of economic sanctions that had crippled its economy.
"We don't think Trump will walk out of the deal despite (his) rhetoric and propaganda," Rouhani told reporters on the sidelines of the U.N. General Assembly gathering of world leaders. He also ruled out the idea of renegotiating the pact.
"Either the nuclear deal remains as it is or it will collapse," he added.
The seven nations that struck the agreement met during the evening along with the European Union, whose foreign policy chief defended the pact and suggested the world did not need another nuclear crisis on top of North Korea's pursuit of atomic weapons and the ballistic missiles to deliver them.
"We already have one potential nuclear crisis. We definitely (do) not need to go into a second one," Federica Mogherini, the EU official, told reporters after the meeting.
Mogherini also said all the nations, including the United States, agreed that all sides were complying with the nuclear agreement and she suggested that if there were non-nuclear issues these should be discussed elsewhere.
Asked if the United States had committed to staying in the pact, she appeared to be at a loss and said: "Another question."
Trump, who on Tuesday called the pact "one of the worst and most one-sided transactions the United States has ever entered into," told reporters 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.
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.
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.
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.
However, an official from a Gulf nation suggested that his country could accept the deal's collapse. Should Trump either not certify Iranian compliance or withdraw from the deal entirely, the Gulf official said: "I think we can live with that." (Reporting by Yara Bayoumy, Parisa Hafezi, Steve Holland, John Irish, Michelle Nichols, Jeff Mason and Arshad Mohammed at the United Nations, Patricia Zengerle, John Walcott and Susan Heavey in Washington, Andrew Osborn in Moscow and Babak Dehghanpisheh in Beirut; Writing by Arshad Mohammed; Editing by Grant McCool and James Dalgleish)