The U.N. management chief says the United Nations headquarters overlooking New York's East River suffered "unprecedented damage" from Superstorm Sandy.

Yukio Takasu told the General Assembly at the end of its first session following the killer storm on Thursday that the most serious damage was from flooding, which affected many basement offices and the cooling system in the main Secretariat building, and caused a small fire.

U.N. safety and security chief Gregory Starr told the 193-member world body that the flooding also affected many electrical components and tore the sheeting off the top of the 39-story Secretariat building.

The U.N. complex is undergoing its first major renovation in 60 years and staff recently started returning to the Secretariat building.