Updated

In her last act at the United Nations, the organization's chief anti-corruption official blasted Secretary-General Ban Ki-moon as a bureaucratic weakling who has reduced the U.N. to a "sad" and perilous state of irrelevance by destroying accountability in his own house.

Inga-Britt Ahlenius, the departing undersecretary general for the Office of Internal Oversight Services, dispatched an end-of-assignment report at the close of her 5-year tenure on Friday that accused Ban of undermining her work to make the world body open and accountable.

Though Ban has frequently touted his own projects to that end, Ahlenius wrote that under his watch, "There is no transparency, there is lack of accountability" at the U.N.

Ahlenius charged that Ban has allowed the U.N. Secretariat — the executive body he controls — to "decay" and "[drift] into irrelevance," reducing the capacity of the U.N. to respond to international humanitarian crises.

"Rather than supporting the internal oversight which is the sign of strong leadership and good governance, you have strived to control it which is to undermine its position," she wrote in a cover letter to her 50-page memo, a copy of which was obtained by the Washington Post. "I do not see any signs of reform in the Organization."

Ahlenius specifically faulted Ban for setting up competing investigations alongside her own and blocking her from appointing officials of her choosing to staff her department.

"Your actions are not only deplorable, but seriously reprehensible," the Post quoted her memo as saying. "Your action is without precedent and in my opinion seriously embarrassing for yourself."

The letter, addressed to Ban and sent to much of the U.N.'s senior brass, represents a rare personal attack on the secretary-general. United Nations documents are generally couched in the gentle and opaque terms of international diplomacy.

U.N. spokesman Martin Nesirky defended the secretary-general's performance Tuesday at a press briefing in U.N. headquarters, telling reporters that Ban came into office striving to improve accountability.

But lawmakers are now pointing to the letter as further evidence that U.S. tax dollars are being wasted at the United Nations.

"The stew of corruption, mismanagement, and negligence long plaguing the UN has reached a boiling point," said Rep. Ileana Ros-Lehtinen, R-Fla., in a press release Tuesday.

"After reading this report, it is even clearer that U.S. taxpayer dollars being shipped off to the UN will continue to be sucked into a black hole unless true reforms are enacted."

Ros-Lehtinen is the author of a bill that would condition U.S. funding and support for the U.N. on the enacting of large reforms.

A replacement for Ahlenius atop the watchdog agency has not been named, though U.N. officials said efforts to improve the body's performance were ongoing.

"The Secretary-General would be the first to say this organization has a long way to go to fully implement the changes that are needed," Nesirky, the spokesman, said.

Click here for more on this story from the Washington Post.