UNITED NATIONS - The UN General Assembly on Thursday will appoint Antonio Guterres as the new secretary-general of the United Nations, setting a shift in motion toward a more reformist, high-profile leadership at the world body.

The former prime minister of Portugal who also served as UN refugee chief will not take over from Ban Ki-moon until January 1, but he already faces high expectations as the world's next diplomat-in-chief.

UN diplomats say they expect Guterres -- the first former head of government in the job -- to shake up the world body and play a more prominent role than Ban, the former South Korean foreign minister known for his low-key style.

French Ambassador Francois Delattre described Guterres as the "best possible captain during this stormy period" of global crises, praising him as a reformer, a unifier and a "humanist with a genuine moral compass."

"Guterres has shown as head of UNHCR that he is a man who seeks human contact and is hands-on," he said.

As Portugal's prime minister from 1995 to 2002 and as UN high commissioner for refugees from 2005 to December 2015, Guterres demonstrated that "he is a reformer", said British Ambassador Matthew Rycroft.

"He is able to do both the outward leadership, the vision, the inspiration, the setting of direction but also the internal reform to drive efficiency, to improve processes and to make the whole more than the sum of its parts," he said.

On Thursday, the 193 member states will adopt a four-paragraph resolution appointing Guterres as the UN's ninth secretary-general for a period of five years.

The 67-year-old socialist politician will deliver his first address as the incoming UN chief before moving in to a temporary office with his transition team in New York, just across the street from UN headquarters.

Some of the buzz generated by Guterres' election stems from disappointment with Ban, a chief many saw as a poor communicator who showed a reluctance to take strong action on the biggest crises -- Syria, Yemen and South Sudan.

"Everyone is very polite about Ban but let's face it... everyone knows that he has not been a strong secretary-general in terms of either internal reform or external leadership," said a Security Council diplomat.

"He has the right instincts on Syria and everything else, but he hasn't had the ability to really drive international opinion on any of these issues," said the diplomat, who spoke on condition of background.

Guterres won unanimous support from the UN Security Council during a vote last week that capped the most transparent campaign ever held at the United Nations for the top post.

Following the council vote, Guterres pledged to serve "the most vulnerable" with humility and pointedly remarked that the deeply divided Security Council could show that it could act quickly.

A few days later, the council voted down two resolutions on imposing a ceasefire in Syria, one of which was vetoed by Damascus ally Russia.

Diplomats said they expect Guterres to spend his first 100 days in office focused on boosting the role of the secretary-general.

"I am not saying he will have a particular plan to bring peace to Syria, but I think that he will be focused on making sure that there is space for the secretary-general to be centrally involved on those biggest issues," said the diplomat.

Another test will come with the appointment of his senior team, with Russia and China making a push for plum posts as head of political affairs or peacekeeping, according to diplomats.