The UN official removed from his post after criticising Afghanistan's presidential election has lashed out at the UN mission in the country.
Peter Galbraith said the $300m mission is now "leaderless", calling the UN special envoy a "terrible manager".
Speaking to BBC News, he also accused Norwegian Kai Eide of failing to act upon evidence of electoral fraud.
Mr Eide responded by saying he had the full backing of the international community and the US administration.
"I feel that Galbraith is on a personal campaign for revenge, after what happened to him," Mr Eide said, adding: "It wasn't me who got fired, it was him."
A UN spokesman rejected Mr Galbraith's accusations, saying public debate about the organisation's credibility in Afghanistan was highly unfortunate.
Mr Galbraith angered Afghan President Hamid Karzai by reportedly calling for a complete recount of the vote.
The election has been overshadowed by widespread allegations of fraud. According to EU election observers, about 1.5m votes - about a quarter of all ballots - cast in August's presidential vote could be fraudulent.
PETER GALBRAITH: KEY DATES
1979-1993: Senior adviser to US Senate Committee on Foreign Relations
1993-1998: First US Ambassador to Croatia, and co-author of Erdut Agreement that ended the war in Croatia
2000-2001: Director of Political, Constitutional and Electoral Affairs for the UN Transitional Administration in East Timor
2003: Resigns from the US government to write The End of Iraq: How American Incompetence Created a War Without End
They say that 1.1 million votes cast for President Karzai are suspicious.
"Once it became clear to Kai Eide that this evidence would be harmful to President Karzai, he ordered us to do nothing with it," he told BBC World News America.
"He had good relations with Karzai, and he became Karzai's man in the United Nations, rather than the United Nations representative to Karzai."
Mr Galbraith also launched a personal attack on his former boss, saying he had lost the confidence of many of the country's political opposition figures and was mistrustful of his own staff.
"So we now have a mission that costs over $300m a year with several thousand employees, that's leaderless," he said.
UN sources say Secretary General Ban Ki-moon decided to end Mr Galbraith's mission after it became clear he was no longer able to carry out his work in Afghanistan. Some Afghan cabinet ministers had said they no longer wanted to work with him.
A UN spokeswoman at the headquarters in the US said the body was "certainly concerned" about the allegations of fraud.
"We're trying to do as much as we can to specify what the UN role exactly was, and what the UN role still is," Michele Montas said from New York.
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