United Nations - Antonio Guterres took the oath of office on Monday as the United Nations' ninth secretary-general, vowing to shake up the world body as it confronts global crises and uncertainty following the election of Donald Trump.During a solemn ceremony at the General Assembly, the former prime minister of Portugal placed a hand on the UN charter and took the oath office administered by the president of the General Assembly Peter Thomson.The first former head of government at the UN helm, Guterres takes over from Ban Ki-moon on January 1 amid ongoing bloodshed in Syria and questions over the US role in the world under a Trump presidency."The organization is the cornerstone of multilateralism, and has contributed to decades of relative peace, but the challenges are now surpassing our ability to respond," Guterres said in an address."UN must be ready to change," he added.The 67-year-old socialist politician said the United Nations must "recognize its shortcomings and reform the way it works" and singled out the failure to prevent crisis as its most serious weakness.The UN's refugee chief for a decade, Guterres was sworn in as Syrian forces were on the verge of retaking the entire city of Aleppo in a potential turning point in the six-year war that has killed 300 000 people."This is a war in which everybody is losing. This became a threat for everybody around the world," Guterres later told journalists. "It is high time to put an end to this nonsense."Guterres vowed to "engage personally" in conflict resolution, signalling a more pro-active approach to the role of secretary-general than under Ban, who led the world body for two five-year terms.The election of Guterres has energized UN diplomats who see him as a skilled politician, able to overcome divisions that have crippled the United Nations, notably over Syria.But Trump's shock election has raised questions over the US role in the world and the new US administration's relationship with the United Nations, as the world body's biggest financial backer.Fear driving decisions The new UN chief will begin work just weeks before Trump moves into the White House. "Fear is driving the decisions of many people around the world," Guterres said, in a reference to the surge of populism that propelled Trump to the White House.Citizens worldwide are losing confidence in their governments and in global institutions, he said, adding that it was "time to reconstruct relations" between leaders and their people.He pledged to engage "with truth" and show the new US administration a "clear will to cooperate in relation to the enormous challenges we will be facing together."Guterres laid out three priorities for change during his five-year term: work for peace, supporting sustainable development and improving internal management.The 71-year-old United Nations has been criticized for its clunky bureaucracy which has at times slowed down the response to global emergencies.Guterres vowed to press ahead with gender parity at the United Nations, saying it was a priority to appoint more women to senior posts.Among the appointments expected soon, Nigeria's Environment Minister Amina Mohammed is tipped to become UN deputy secretary-general, diplomats say. An engineer by training and practising Catholic, Guterres fought for migrants' rights over a decade as UN High Commissioner for Refugees from June 2005 to December 2015.He served as prime minister of Portugal from 1995 to 2002, anchoring his country to the European Union and working to raise living standards.