Media playback is unsupported on your device Media caption New UN Secretary General Antonio Guterres says ending the conflict in Syria is his top priority

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.

He told the BBC that ending the civil war in Syria would be his biggest challenge.

"I believe it is the international community's first priority is to be able to end this conflict and use this momentum created by it to try to address all the other conflicts that are interlinked," he told the BBC's chief international correspondent Lyse Doucet.

He said the world was facing a dangerous time and he wanted to see people across the globe working together to achieve a safer future.

"I hope people will understand that it's better to put aside different opinions, different interests and to understand that there is a common, vital interest to put an end to these conflicts, because that is absolutely central if you want to live in a world where a minimum of securities are established, where people can live a normal life," he said.

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Mr Guterres, who trained as an engineer, entered politics in 1976 in Portugal's first democratic election after the "Carnation Revolution" that ended five decades of dictatorship.

As head of the UNHCR refugee agency from 2005 to 2015, he led the agency through some of the world's worst refugee crises, including those in Syria, Afghanistan, and Iraq.

During that time, he repeatedly appealed to Western states to do more to help refugees fleeing the conflicts.

Mr Guterres' nomination came despite a concerted effort to appoint the UN's first female secretary general. Of the 13 candidates, seven were women, among them Unesco director-general Irina Bokova from Bulgaria, and Helen Clark, 66, a former prime minister of New Zealand and current head of the UN development programme.

Mr Guterres told the general assembly: "The dramatic problems of today's complex world can 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, working as a convenor, a mediator, a bridge-builder and an honest broker to help find the solutions that benefit everyone involved."

Image copyright AFP Image caption Mr Guterres will become the world's top diplomat on 1 January

Mr Ban told the assembly that his successor was well-known in diplomatic circles as a man of compassion.

"He is perhaps best known where it counts most - on the frontlines of armed conflict and humanitarian suffering," Mr Ban said.

"His political instincts are those of the United Nations - co-operation for the common good and shared responsibility for people and the planet."

What does a UN secretary-general do?

The Security Council - with five of its members wielding vetos - is the most powerful body in the UN.

While not as powerful, the secretary-general serves as the organisation's top diplomat and chief "administrative officer".

It has been described as the most impossible job in the world, says the BBC's diplomatic correspondent James Landale. The secretary-general has to run an unwieldy bureaucracy and manage the competing demands of the world's big powers, he adds.

A key requisite of the role is to step in both publicly and privately to prevent international disputes from escalating. The post lasts for five years and is limited to a maximum of two terms.