Antonio Guterres calls for changes at UN as he takes office
United Nations Secretary-General-designate Mr. Antonio Guterres of Portugal is sworn in by President of the UN General Assembly Peter Thomson (R) at UN headquarters in New York, U.S., December 12, 2016. REUTERS/Lucas Jackson TPX IMAGES OF THE DAY

Antonio Guterres, the former prime minister of Portugal, took oath on Monday as the ninth secretary-general of the United Nations at a gathering at the General Assembly.

The 67-year-old leader, who takes over from outgoing UN chief Ban Ki-moon on January 1, made a personal pledge to usher in peace in conflicts zones including Syria.

"This organisation is the cornerstone of multilateralism, and has contributed to decades of relative peace, but the challenges are now surpassing our ability to respond ... UN must be ready to change," ," Guterres said after being sworn in.

The socialist politician, who is the first former head of state to assume the leadership of the 71-year-old world body, has a tough task on his hands with conflicts in various parts of the world and as a Donald Trump presidency set to begin in the US.

Guterres has hinted at taking up a more active role in mediation in conflicts, something the UN diplomats have already warmed up to. "He's looking for a big shake-up, reshuffle ... He's looking to create ... a different feeling, with the under-secretary-generals much more part of a collective leadership of the U.N," a senior UN diplomat speaking on condition of anonymity told Reuters.

Guterres, who was the UN High Commissioner for Refugees from 2005 to 2015, said he will be hands-on in Syria and other flash points. "From the acute crises in Syria, Yemen, South Sudan and elsewhere, to long-running disputes including the Israeli/Palestinian conflict, we need mediation, arbitration as well as creative diplomacy.... As part of my good offices I am ready to engage personally in conflict resolution where it brings added value," Guterres said in the General Assembly.

Guterres is expected to appoint Nigerian environment minister Amina Mohammed as his deputy secretary-general, the news agency reported. He will also name a woman as his chief of staff.