The US special envoy to Afghanistan had a heated row with President Hamid Karzai over the alleged use of fraud in last week's election, according to reports.
Richard Holbrooke is said to have complained to the President about the use of ballot-stuffing and suggested that a second round to the elections would increase the credibility of the Afghan democratic process.
Mr Karzai reportedly reacted angrily to the accusations last Friday.
A spokeswoman for the US embassy in Kabul denied there had been any shouting or that Mr Holbrooke had stormed out, according to the BBC.
Michael Semple, former deputy head of the EU mission to Afghanistan, said it was now "absolutely clear" there had been "massive fraud".
Asked on BBC Radio 4's Today programme whether it would be better for the future of Afghanistan if a second round was held, he said: "That's the point that we understand that Ambassador Holbrooke made, however we also understand that President Karzai disagreed with that fervently.
"And some of his supporters have been deliberately leaking the information about the 68% or 72% which is why there probably was a battle royale in the presidential palace on the 21st (August)."
Last week the President's aides briefed that Mr Karzai had won the election with an overwhelming 68% share of the vote.
But preliminary results announced by the Independent Election Commission (IEC) after votes from 10% of polling stations had been counted showed Mr Karzai has 40.6% of the vote, with Mr Abdullah on 38.7%.
A candidate needs to secure 50% of the votes to avoid a run-off contest against his leading rival.
