UN staff are expected to make an unprecedented vote of no confidence in Secretary-General Kofi Annan, union sources say, after a series of scandals tainted his term in charge of the world body.

The UN staff union, in what officials said was the first vote of its kind in the almost 60-year history of the United Nations, was set to approve a resolution withdrawing support for Annan and senior UN management.

Staffers said the trigger for the no-confidence measure was an announcement this week that Annan had pardoned the UN's top oversight official, who was facing allegations of favouritism and sexual harassment.

The union had requested a formal probe into the official, Dileep Nair, after employees accused him of harassing staff and violating UN rules on the hiring and promotion of workers.

Top UN spokesman Fred Eckhard announced on Tuesday that Nair had been exonerated by Annan "after a thorough review" by the UN's senior official in charge of management, Catherine Bertini.

Veteran UN staff said this was the first time that employees had risen up to make a vote of no confidence in a sitting secretary general.

"Kofi Annan is surrounded by corruption, a gang of criminals responsible for some of the worst things that happened to mankind in the 20th century," said one angry staffer, referring to the Rwanda massacres.

"It's possible that he doesn't know directly what has gone on," said the employee, who has worked for the United Nations for two decades. "But that's no excuse."