Former Portuguese Prime Minister Antonio Guterres has been officially appointed as the next UN secretary-general.

He will become the world's top diplomat on 1 January when Ban Ki-moon's second five-year term ends.

Mr Guterres, 67, who led the UN refugee agency UNHCR for 10 years, was chosen from among 13 candidates last week.

Antonio Guterres pledged on Thursday to be an "honest broker" and "bridge-builder" on global issues after the United Nations General Assembly unanimously voted to appoint him as the next secretary-general of the world body.

The former prime minister of Portugal and UN refugee chief will replace outgoing Secretary General Ban Ki-moon on January 1 as the world faces conflicts from Syria and Yemen to South Sudan, refugee and humanitarian crises, climate change and development challenges.

Guterres was welcomed with applause after the vote as he told the General Assembly that he was "fully aware of the challenges the UN faces and the limitations of the secretary-general."

"The dramatic problems of today's complex worldcan only inspire a humble approach, one in which the secretary-general alone neither has all the answers nor seeks to impose his views, one in which the secretary-general makes his good offices available ... to help find solutions that benefit everyone involved," he told the chamber of 193 member-states. The 67-year-old socialist politician will serve a five-year term.

On Syria, the most pressing crisis facing the UN, Guterres said world powers must overcome differences to resolve a burning conflict that has killed nearly 400,000 people and displaced millions as it remains a threat to world peace.

"Whatever divisions might exist, now it's more important to unite," Guterres told reporters after the vote that will make him the ninth secretary-general of the world body. "It's high time to fight for peace."

His call comes as the United States and Russia, alongside Turkey, Qatar and Saudi Arabia were set to meet in the Swiss city of Lausanne on Saturday to discuss the Syrian crisis. Washington will then meet with its European partners on Sunday in London in a bid to give renewed momentum to talks to end the conflict after two recent Security Council resolutions on Syria were squashed.

Guterres also said "diversity can bring us together not drive us apart," warning that the world community must unite against terrorism and a rise of xenophobic populism.