The White House on Friday condemned any evidence of fraud from the Afghan election, after details emerged of a "testy" meeting between a top US envoy and President Hamid Karzai.
Revelations about the encounter were revealed as concern mounted about alleged irregularities in the election, and US combat deaths for the month hit 46, making August the deadliest month of the war so far for American soldiers.
President Barack Obama's National Security Council declined to elaborate on reportedly fraught talks between envoy Richard Holbrooke and Karzai on the day after the August 20 polls but promised continued vigilance over electoral irregularities.
"We condemn any acts of fraud; it is important that the outcome of these elections reflect the will of the Afghan people," NSC spokesman Michael Hammer said.
"Robust mechanisms exist for dealing with incidents of fraud, through the IEC and the Electoral Complaints Commission and we need to be patient and allow those bodies to do their work thoroughly.
"We will continue to encourage Afghan authorities to follow the comprehensive anti-fraud measures established in order to protect the integrity of the election process and ensure that the election results are credible."
A separate US official with knowledge of the meeting between Holbrooke and Karzai said on condition of anonymity that the US envoy brought up allegations of widespread vote-rigging.
"It was a difficult meeting and there were some sharp exchanges in it," the official said in Washington.
"The thrust of the meeting was to respect the electoral process, let it take its course and be patient and to respect the results, whatever they are," the US official said.
Karzai's main opponent Abdullah Abdullah has alleged massive fraud and ballot stuffing and made formal complaints to the Election Complaints Commission, which received 790 allegations of fraud on election day alone.
Analysts said that by making public the fractious nature of the exchange between Holbrooke and Karzai, the US administration has revealed its concern that fraud allegations and low turnout would damage the election's credibility.
But any judgment that the Afghan elections were not free and fair would complicate Obama's task of retaining public support for the Afghan operation, with US troop deaths rising.
Obama, who is currently on vacation on the island of Martha's Vineyard, initially said the elections seemed to have been successful, but since then more claims of fraud have surfaced.
Earlier Friday, a bomb blast killed a US soldier, in the latest deadly attack on American forces.
The soldier's death brought the American military death toll to its highest point for a single month since the conflict began in 2001, a US official said.
A total of 46 troops have been killed in Afghanistan so far this month, higher than the previous monthly record set in July with 45 deaths.
After less then nine months, 2009 is already on record as the deadliest year for all foreign troops in Afghanistan since the 2001 US-led invasion ousted the extremist Taliban regime and sparked an increasingly virulent insurgency.