"Cooperation between the United States and the United Nations is an essential pillar of international relations," Guterres said in a statement that congratulated Trump on his victory.

"The United Nations stands ready to work constructively with the incoming administration to address the dramatic challenges our world is facing."

Guterres began his role leading the United Nations just as Trump entered the White House in January 2017.

One of his main tasks was contending with Trump's unilateralist policies, which struck at the core of the UN's work of bringing countries together and encouraging international cooperation.

Most observers see Guterres as having largely succeeded, enabling the UN to survive the Republican's first term.

That period saw the United States slash its contributions to the UN peacekeeping budget and to key UN agencies like the World Health Organization -- whose chief Tedros Adhanom chimed in on X to call the partnership between the WHO and Washington "vital."

Richard Gowan of the International Crisis Group told AFP that "Guterres and his team have been bracing and planning for Trump's return for quite some time. They expect the US to reduce funding to many parts of the UN quite drastically."

The UN is already facing budget pressures, and some observers are particularly concerned about funding for programs related to reproductive rights.

- 'Fiercely anti-UN' -

During Trump's first term, the United States also withdrew from the culture and education agency UNESCO and the Paris climate accord.

The fight against global warming remains a priority for Guterres, while Trump has called it a hoax.

"The fact that they had different opinions about a number of issues was clear to all," Guterres's spokesman Stephane Dujarric said Wednesday, while also describing "very good relations" between the UN chief and Trump.

However, the world has changed in the past four years, particularly over the last year, with the escalation of conflicts in the Middle East.

The UN chief has been outspoken in his warnings about the "nightmare" facing Palestinians in Gaza, devastated by Israeli reprisals after Hamas attacks on October 7, 2023.

Due to his criticism of Israel, "many members of the Republican Party in Congress are fiercely anti-UN," making it harder for Guterres to manage relations with Washington, Gowan said.

As Israel continues to target the UN and its chief, Israeli Ambassador Danny Danon congratulated Trump on his presidential victory, calling him an ally for Israel at the UN.

"I'm confident that President Trump and his team will continue to stand with Israel at the United Nations against the hypocrisy and the hate we are facing in this building," Danon said in a video posted on X.

It remains to be seen who Trump will appoint as US representative to the world body.

During his first term, Trump initially nominated Nikki Haley, known for her outspoken style, before she resigned.

Haley later became his rival in the 2024 Republican primaries.