UN Secretary-General Antonio Guterres on Monday congratulated Joe Biden on winning the US presidency -- and the American people for a "vibrant exercise of democracy."

Biden was projected as the winner of the bitterly contested November 3 election on Saturday by US media, though President Donald Trump has yet to concede.

"The Secretary-General congratulates the American people for a vibrant exercise of democracy in their country's elections last week," spokesman Stephane Dujarric told reporters.

"He congratulates the President-elect and Vice President-elect and reaffirms that the partnership between the United States and the United Nations is an essential pillar of the international cooperation needed to address the dramatic challenges facing the world today."

Dujarric did not explain why Guterres waited two days to offer his congratulations to Biden and his running mate Kamala Harris.

From Saturday, dozens of world leaders and heads of international organizations offered their well wishes to the Democratic duo.

Diplomats who spoke to AFP on condition of anonymity said Guterres, whose term ends at the end of 2021, was not planning to ask for a second term if Trump were reelected.