The senior UN envoy in Afghanistan has given his one of his strongest indications yet of concern over the conduct of the country's election.
Kai Eide acknowledged that "widespread fraud" affected the presidential vote.
Responding to accusations that he covered up the extent of malpractice, he said fraud was "significant" but any effect on the result remained unclear.
Allegations made by his former deputy were an attack on his dignity and hurt the entire electoral process, he said.
Peter Galbraith - dismissed from his post at the end of September - has labelled the Kabul-based mission "leaderless" and called Mr Eide a "terrible manager".
He also called for a complete recount of the Afghan vote, reportedly angering President Hamid Karzai - whose supporters have been most closely linked with irregularities.
Speaking in Kabul accompanied by ambassadors from the US, the UK and France, Mr Eide, a Norwegian, was clear in his judgement that fraud had taken place during August's election.
The allegations made against me... have affected the entire election process
Kai Eide UN envoy in Afghanistan
"It is true that in a number of polling stations in the south and the south-east there was significant fraud," he said.
"There is no way to know at this stage what the level of fraud is. I do not know, nobody else knows.
"The extent of that fraud is now being determined," he added, calling any attempt to put a figure on it as "pure speculation".
According to EU election observers, about 1.5m votes cast in August's presidential vote - about one quarter of all ballots - could be fraudulent.
Mr Eide said allegations against him by Mr Galbraith had been hugely damaging.
"The allegations made against me by my former deputy have not only been personal attacks against me and my integrity, but they have been attacks that have affected the entire election process."
The BBC's Martin Patience in Kabul says it was significant that Mr Eide came out fighting.
It also appears that the international community, in the shape of ambassadors from Western countries, was closing ranks behind their man in Afghanistan, our correspondent says.
The Afghan Electoral Complaints Commission and the Independent Election Commission have almost completed an investigation of ballot boxes identified as suspicious, the UN has said.
Mr Galbraith was said to have been stood by the UN secretary general
Mr Karzai leads preliminary results with about 55% of the vote, considerably ahead of his nearest rival Abdullah Abdullah, who has 28%.
Separately on Sunday Mr Karzai was reported as saying the delay in announcing an official winner was damaging Afghanistan.
"Day by day, Afghanistan faces more security problems, there is a decrease in the national income, there is an increase in insecurity across the country, [and] there is a negative impact on daily business," the Associated Press reported him as saying.
Disputes about the electoral process and its aftermath caused differences between Mr Eide and Mr Galbraith, his American deputy, to erupt in public.
Mr Galbraith left Afghanistan following an apparent row with Mr Eide over the UN's approach to the electoral fallout.
He was then removed from his position by the UN before going public with concerns over Mr Eide's leadership.
He alleged Mr Eide covered up the extent of the fraud to maintain good relations with Hamid Karzai.
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