Secretary-General Kofi Annan (search) appointed a new chief of staff Monday in the first of what he described as a "series of changes" among senior U.N. personnel.

Mark Malloch Brown (search), currently the administrator for the U.N. Development Program, will begin his new job as Annan's right-hand man on Jan. 19. He replaces Iqbal Riza (search), who retired Dec. 22.

Annan said he intends to make other new senior appointments but declined to give details.

"This is the first in a series of changes," he said.

Because of current tsunami crisis in South Asia, Malloch Brown, a British citizen, will continue in his role as UNDP administrator until a replacement could be found, Annan said.

"There is clearly a need for continuity at this critical moment," he said.

Annan said he had a long list of candidates to head the UNDP but those candidates do not include an American because U.S. citizens held the position for more than 40 years before Malloch Brown's appointment.

"The other two I think should go to other regions and other nationalities. I do intend to appoint some other nationality and not an American," he said.

Annan said he had known Malloch Brown for 25 years since working with him at the U.N. High Commission for Refugees.

"Mark is an immensely capable leader and manager." he said. "In his new role he will assist me ... in developing and implementing major initiatives to improve the performance and management of the United Nations."

Malloch Brown said the United Nations faced several challenges in coming months in addition to coordinating relief and reconstruction aid for countries hit by the Dec. 26 tsunami.

Those include an initial report into alleged corruption in the U.N. oil-for-food program, which is expected this month, and the U.N.'s push for progress on targets to reduce poverty and illiteracy and improve health care worldwide.

He also noted that "it has been the subject of wide commentary that staff morale has not been at its highest at this time."

Annan is hoping to spend his last two years at the helm of the United Nations pressing for a sweeping overhaul of the world body's nearly 60-year-old machinery so it can tackle global security threats in the 21st century.

Riza, a Pakistani, worked for Annan for seven years.

"He has offered wise counsel and advice throughout, through thick and thin. He well deserves his long-deferred retirement but I am sure I will continue to rely on his advice," he said.