The UN has called for a crackdown on Afghan poll fraud, amid mounting concerns about irregularities from last month's presidential election.
UN envoy Kai Eide said results from all ballot boxes in areas where there is any evidence of fraud must be annulled.
The BBC has learned that the US ambassador in Kabul and a senior UN official met President Hamid Karzai on Monday night to discuss their concerns.
In the most recent partial results, Mr Karzai was close to outright victory.
The 20 August vote has been overshadowed by claims of mass fraud and ballot-box stuffing.
The BBC's Chris Morris in Kabul says more results are due to be declared on Tuesday, but many senior international officials believe they could be fraudulent.
Afghan election officials at the weekend annulled results from 447 poll centres, amounting to about 200,000 ballots.
But there are fears the irregularities were much more widespread, says our correspondent.
America's ABC news has quoted Western officials as saying the number of questionable votes could be more than one million.
The latest poll results on Sunday showed Mr Karzai near to the 50% threshold needed to avoid a run-off ballot, with three-quarter of votes counted.
But the Afghan Independent Election Commission has received more than 650 major serious fraud complaints.