UN officials said Mark Malloch Brown would replace Iqbal Riza, 70, who announced his retirement as chief of staff two weeks ago, about the same time as two other top UN officials. The United Nations described the moves as coincidental year-end changes.

The appointment comes, however, after a reported meeting a month ago where Mr Annan was advised to attend to the poor relations with the US administration due to concerns over the Iraqi oil-for-food program, accusations of criminal activities by peacekeepers in Congo, cronyism and other issues.

Mr Annan also said the appointment of the new chief of staff was the first in a series of changes among senior personnel.

The openings give Mr Annan an opportunity to put in place UN reforms he has advocated and tackle the damage caused to the world body over corruption in the oil-for-food program.

That program permitted Iraq under Saddam Hussein to sell oil in spite of an economic embargo provided the proceeds were used for food and medicine.

The United Nations is also investigating peacekeepers in the Democratic Republic of the Congo for rape and prostitution. The challenges come as the body is coordinating massive relief efforts to victims of the Asian earthquake and playing a key role in organizing elections in Iraq.

Malloch Brown, a 51-year old Briton, whose official title is administrator of UNDP, is credited with a reform of the agency, which has programs in 166 nations. He took office in 1999 after serving as a World Bank vice-president in charge of external affairs and public relations.

The other two officials who have decided to leave are Catherine Bertini, an American undersecretary-general for management and Jean-Pierre Halbwachs of Mauritius, the UN controller.

Ms Bertini spent 10 years as executive director of the Rome-based UN World Food Program before coming to New York in January 2003. She is expected to leave in the spring and said she informed Mr Annan in September she wanted to pursue other career opportunities.

Mr Halbwachs had told his staff he wanted to take early retirement in February when he turns 55, after 32 years in the world body, UN spokesmen said.

A group of friends outside of the United Nations met Mr Annan on December 5th in the apartment of former UN Ambassador Richard Holbrooke to discuss ways of rescuing the world body and Mr Annan’s reputation after hard-liners in Congress kept up a barrage of criticism over the oil-for-food program, the New York Times reported.

They argued, according to participants, that Mr Annan had to repair relations with Washington, where some in the Bush administration thought he and the United Nations had worked against President Bush’s re-election, the newspaper said.

Participants also said Mr Annan needed to tackle his own bureaucracy, where staff union officials have said his office protected high officials from misconduct, the newspaper reported.

Among the most visible cases was the retention of Dutchman Ruud Lubbers, the UN High Commissioner for Refugees, accused of sexual harassment and alleged to have intimidated witnesses against him.

Another is UN watchdog Dileep Nair, alleged to be favouring Indian nationals and accused by the union of corrupt practices in his own office.

Mr Annan was reported to have listened but not made any comment.

In the months ahead, Mr Annan is faced with “an extraordinary and challenging agenda that also is an opportunity for the UN to demonstrate again that with all its faults and shortcomings it is an indispensable force on the world scene,” said Mr Holbrooke.

The secretary-general must deal with the oil-for-food and sex scandals, the tsunamis in Asia and Africa, elections next week among the Palestinians to choose a successor to Yasser Arafat, and conflict in the western Sudanese region of Darfur, Mr Holbrooke said.

On Thursday, Mr Annan is due to join Mr Powell at an international donor’s conference for tsunami victims in Jakarta, Indonesia.