U.N. Secretary General Kofi Annan's top advisor is leaving his job at the world body just before the release of a report into the troubled Oil-for-Food (search) program.

Iqbal Riza (search), a former Pakistani diplomat who has worked at the United Nations for more than 25 years, is retiring effective Jan. 15, the secretary general’s office announced Wednesday.

In a statement, Annan said he agreed to Riza's departure "with very mixed emotions,'' saying "he... has always provided me with wise and trusted counsel.''

"Mr. Riza has, throughout these years of service to the United Nations, carried out his duties with unflinching dedication to the ideals and principles of the organization,'' Annan said.

Riza, 70, joined the United Nations (search) in 1978 and served in Iran, Iraq, Nicaragua, El Salvador, Bosnia and the Department of Peacekeeping Operations before becoming the head of Annan's executive office in 1997, with the rank of undersecretary-general.

Riza is considered a forceful defender of the United Nations and has been Annan’s closest high-level aide. He reportedly has urged Annan not to share with Congress documents about Oil-for-Food that lawmakers have asking for.

The U.N. Staff Union also has been upset over Riza’s and senior management’s handling of sexual misconduct investigations.

U.N. insiders think Riza’s departure will signal a larger change in the top jobs at the world body at a time when Paul Volcker is set to release his independent report on the Oil-for-Food program.

FOX News' Eric Shawn and The Associated Press contributed to this report.