Current results have President Hamid Karzai was close to the 50% threshold needed for an outright win in his re-election bid. But with reports of phony ballots and polling stations growing, U.S. ambassador Karl Eikenberry met Karzai on Monday night to discuss his concerns.
The UN-backed Election Complaints Commission said there should be a recount where any single candidate received more than 95% of valid votes, in any polling station where more than 100 votes were cast.
Karzai's nearest rival, Abdullah Abdullah, who has claimed the vote amounted to "state-engineered fraud" - was about 17 percentage points behind.
The fraud controversy comes as Washington considers whether to send thousands more troops to the country, where violence has hit a record high eight years on from the US-led invasion.
Get more: BBC