UNITED NATIONS (Reuters) - French President Emmanuel Macron cast himself as the antithesis of Donald Trump at the United Nations on Tuesday, a position that could enable him to play the honest broker in international conflicts.  A few hours after the U.S. president pugnaciously defended his unilateralist America First doctrine and launched scathing verbal attacks on North Korea and Iran, Macron took to the same stage to advance a diametrically opposed stance.   Multilateralism is the most efficient way to face global challenges. It is the realization of a vision of the world that protects us,  Macron said in his first address to the U.N. annual gathering of world leaders in New York.  The response to Trump was barely veiled when he added minutes later that multilateralism was  also believing that the United Nations has all its legitimacy to act and maintain international balances.  The two men, who had a warm exchange on Monday ahead of a bilateral meeting, appear to have little in common on paper, but since his election in May, Macron, 39, has nurtured a close relationship with the 71-year-old businessman-turned-politician who entered the White House eight months ago. Despite Trump s unpredictable foreign policy, Macron has sought not to isolate Washington, sensing an opportunity to sway its thinking and elevate the role of France, a nuclear power and permanent member of the U.N. Security Council, in global affairs. As part of that, Macron has also sought to improve ties with Russia despite its tense relationship with the West.  IRAN-US MEDIATION  While Trump on Tuesday called a 2015 nuclear deal with Iran an  embarrassment  and the  worst ever , Macron staunchly defended it as  good  and labeled its opponents as  irresponsible.   That President Trump thinks that this agreement is not perfect and doesn t protect enough is an argument that I can hear, but I asked him what was his alternative proposal. I didn t understand it,  Macron later told reporters. He quickly followed the criticism with an acknowledgment that Trump s fears were not completely irrational.   I m proposing additional elements (to the deal) that are independent, but that will alleviate concerns that the United States and countries in the region may have,  Macron said. French and European diplomats said Macron had also had a frank exchange with Iran s president Hassan Rouhani on Monday, warning him that Tehran should stop destabilizing the region and stoking Trump s furor.   JAW-JAW BETTER THAN WAR-WAR Despite little prospect of getting Iran and the United States around the same table to push a new peace initiative to end a six-year-old Syria civil war, Macron suggested he would try to mediate between the two rivals.  If we don t resolve the Syrian problem with Iran around the table, then we will not have an efficient response because Iran today is among the powers that have an influence on the ground,  Macron said. Paris is trying to set up an international contact group on Syria to revive stalled peace talks in Geneva, but Trump s anti-Iran stance has partly complicated those efforts.  Noting the U.S. refusal today, I want that the contact group advances and that the United Nations and France can serve as intermediaries with Iran,  Macron said.  Macron, France s youngest leader since Napoleon two centuries ago, used the U.N. platform to stress that war was truly a last resort and that past mistakes had shown that. Trump had earlier escalated his standoff with North Korea over its nuclear challenge, threatening to  totally destroy  the country of 26 million people and mocking its leader, Kim Jong Un, as a  rocket man.   Multilateralism must do everything to avoid war. Every time we have forgotten this and tried to get a short-term satisfactory solution without a diplomatic roadmap we failed and that was the case in Iraq and Libya,  Macron said.   With North Korea we are not there. I think it is untimely to put forward the threat of war,  he added, urging better implementation of sanctions to pressure Pyongyang back to peace talks. Despite clear differences on policy, Macron has repeatedly emphasized that he shares certain objectives with Trump, making crushing Islamic State and countering global terrorism leading priorities.  We have an exemplary relationship with the United States in fighting terrorism in Africa and the Middle East,  Macron said.  While Trump irked Macron with his decision to pull out of a landmark international accord reached in Paris in 2015 to fight climate change, the French leader has not given up on trying to get Trump to reconsider.   We have a disagreement on climate. President Trump said he wanted to leave the deal, but on a legal basis he still needs to put that into action so it s not a legal reality yet.  The Paris accord is a basis.. I will make it live. I continue to talk to the United States with the hope that it comes back into the international fold and clarifies its vision of things.  