UNITED NATIONS — With appointments of new officials and promises to fill more top positions as soon as this week, Secretary-General Ban took the reins at the United Nations from Kofi Annan yesterday.

Aside from shifting the start of the U.N. workday to 9 a.m. from 9:30 — a change dreaded by U.N. staffers — Mr. Ban has kept mum about his plans for most institutional changes, delaying announcements on his top appointments until he takes office today.

Over the holiday weekend, the new secretary-general and his top Korean aides, as well as members of a team of U.N. officials that helped him in the transition period, met for a two-day strategy retreat at a Long Island resort, Greentree Estate, to finalize the major decisions on new appointments.

Tomorrow Mr. Ban is expected to name his top aide to manage the U.N. bureaucracy, as well as his choice to lead the Office for the Coordination of Humanitarian Affairs. By Friday he will announce his much-anticipated selection for deputy secretary-general.

Washington has lobbied in recent months for an American to be chosen to lead the prominent Department of Political Affairs, a position now held by Ibrahim Gambari of Nigeria. If successful, America might lose its claim to the management office, which it traditionally has controlled.

A Briton could now take the helm of OCHA, the humanitarian office that has attained a high profile under the leadership of Jan Egeland of Norway, although Britain is also competing with America for the political affairs position.

Mr. Ban has said he hopes to name a woman from a developing country as deputy secretary-general. The leading candidates are from Jordan, Saudi Arabia, and Nigeria.

On his first day of work today, Mr. Ban will meet with U.N. staffers and top officials “from the bottom floor up” and speak with top aides around the world, his new chief spokeswoman, Michele Montas, told The New York Sun.

This week, Mr. Ban is expected to contact all U.N. undersecretaries-general and assistant secretaries-general to inform them whether their services will be needed after the end of February, the expiration of most top U.N. contracts.

Other than the new appointments, Mr. Ban, the former South Korean foreign minister, is expected to work closely with at least three of his compatriots, with Kim Won-soo taking a leading position, and Yoon Yeocheol and Mr. Ban’s one-time spokeswoman, Choi Soung-ah, also securing roles.

The appointment of Ms. Montas — a former Haitian journalist who was featured with her late husband, Jean Dominique Montas, in the Jonathan Demme film “The Agronomist” — was announced Sunday. Also named was a special adviser in Mr. Annan’s office, Vijay Nambiar, who will serve as Mr. Ban’s chief of staff.

Mr. Nambiar was once India’s deputy national security official and has known Mr. Ban for decades. With Mr. Nambiar’s appointment, Mr. Ban has secured the support of India, a major U.N. power, as well as that of China, with which Mr. Nambiar maintains close ties after serving as ambassador to Beijing in the late 1990s.

Last summer, Mr. Annan sent Mr. Nambiar on a Middle East mission along with two other top officials, regional coordinator Alvaro de Soto and troubleshooter Terje Roed-Larsen. The three envoys reportedly clashed constantly about policy and personal issues, and several sources familiar with the trip described it as a “disaster.”

As chief of staff, Mr. Nambiar will replace Alicia Barcena, a Mexican and protégé of ex-U.N. official Maurice Strong. Mr. Strong was forced to resign last year under a cloud of suspicion in the oil-forfood scandal.

Ms. Barcena, who represented the United Nations on Mr. Ban’s transition team, is expected to be named undersecretary-general for management, a post vacated last month by an American, Christopher Burnham, Reuters reported yesterday. There was no confirmation of the appointment yesterday.