VIENNA - Any collapse of the 2015 nuclear deal between Iran and major powers would be a "great loss," the head of the UN atomic agency policing the accord said on Monday, alluding to a US threat to pull out of it.US President Donald Trump has threatened to pull out of the deal unless Congress and European allies help "fix" it with a follow-up agreement.International Atomic Energy Agency chief Yukiya Amano, whose agency is monitoring Iranian compliance with curbs on its disputed nuclear activity imposed by the agreement, has long called the deal a "net gain" for nuclear verification, since it has provided the IAEA with more thorough oversight of Iran.But in a speech on Monday to a quarterly meeting of the IAEA's Board of Governors, he went further, evoking the possibility of the deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), unraveling."The JCPOA represents a significant gain for verification," Amano said, according to a text of his speech published by his agency. "If the JCPOA were to fail, it would be a great loss for nuclear verification and for multilateralism."Amano said Iran was implementing its commitments under the deal, which also lifted painful economic sanctions against the Islamic Republic. He confirmed the findings of a quarterly, confidential IAEA report on Iran issued last month.As mentioned in the report, the IAEA has requested clarification from Iran about its plans to develop nuclear marine propulsion, Amano said, suggesting the IAEA has still not heard back from the Islamic Republic."The agency has requested Iran to provide further clarifications regarding its plans relevant to the development of the nuclear fuel cycle related to naval nuclear propulsion," Amano told the closed-door meeting of 35 member states.Iran has long said publicly that it plans to develop nuclear propulsion for naval vessels but analysts and diplomats say remains a distant prospect.It formally notified the IAEA of that intention in January in what was widely seen as a diplomatic warning shot aimed at the Trump administration, which reversed a policy of detente with Iran introduced by his predecessor Barack Obama. But the move prompted the IAEA to ask what exactly Iran's plans are.France's foreign minister visited Iran on Monday on a delicate mission to affirm European support for the nuclear deal that opened up Iran's economy, while echoing U.S. concern about Tehran's ballistic missile program and role in Middle East conflicts.