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

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

The water that rushed across a major highway along the river also caused a small fire in a circuit breaker in an electrical panel in the basement, he said. It was put out almost immediately by the water that continued to rise early Tuesday morning.

U.N. safety and security chief Gregory Starr told the 193-member world body that the flooding affected many electrical components and the U.N. computer system, which was expected to return to operation late Thursday. It tore temporary sheeting off the top of the General Assembly building and destroyed a tent over its entrance, he said.

The sprawling U.N. complex is undergoing its first major renovation since it opened 60 years ago, at a cost of about $2 billion.

Some of the 3,000 staffers forced to move to temporary offices around Manhattan recently started returning to the 39-story Secretariat building. Another 2,000 have remained on the 17-acre (6.8-hectare) site, including Secretary-General Ban Ki-moon and other senior officials, who work in a temporary building constructed just north of the main one, which was not affected by Sandy.

New York architect Michael Adlerstein, the project's executive director and a U.N. assistant secretary-general, said that the schedule for completing the renovations has not been affected by Sandy. The work is to be finished before the annual gathering of world leaders at the General Assembly in September 2014.

Starr said the storm surge from Sandy sent water into the third basement, completely inundating the fourth basement where the cooling plant, some printing facilities and other offices are located.

During a tour of the U.N.'s third basement, an AP correspondent saw staff members mopping floors and some areas nearest the East River still flooded. Cars parked in that basement had flyers on their windshields saying the garage had been inundated with three feet (a meter) of water so "PLEASE test your car brakes several times before driving away."

Among the hard-hit areas in the third basement were the U.N. receiving office, mail and supply rooms, where staffers said perhaps hundreds of thousands of dollars' worth of computers, other equipment, paper and supplies were damaged and likely unusable.

"The most serious damage is due to flooding," Takasu told reporters. "The core infrastructure of all buildings in headquarters is assessed as being intact. I think this is good news."

Starr said there was another piece of good news — no damage has been reported to the U.N.'s valuable archives and library, though the print shop "took a lot of damage."

Takasu said it was too early to put a price tag on the damage, adding that the most costly item that needs replacement is the cooling system, which was insured.

Despite the damage to U.N. headquarters, Secretary-General Ban Ki-moon pledged U.N. assistance to the U.S. and Caribbean nations affected by Sandy in their recovery and rebuilding efforts, U.N. deputy spokesman Eduardo del Buey said.

The U.N. chief said the United Nations will also support efforts to strengthen programs to reduce the risk of global disasters and prepare the world "for a safer tomorrow," del Buey said.