Afghan authorities are facing growing pressure to open a full anti-fraud probe into the presidential election as EU observers described polling data as "very worrying" and one candidate confirmed he would reject the result.
Abdullah Abdullah, who is reported to be losing the vote count, dismissed plans for a partial audit of the ballots and said he would not accept the outcome due for release Monday - pitching the election into further turmoil.
The June 14 election run-off has been mired in allegations of cheating, with Abdullah and his poll rival Ashraf Ghani at loggerheads in a dispute that threatens Afghanistan's first democratic transfer of power.
US-led allies who have fought a 13-year war against Taliban militants and spent billions of dollars in aid are eager to avoid a prolonged power struggle in Kabul.
But a risky political stalemate looms at a time when Afghan security forces are taking on the battle against the resilient Taliban insurgency as NATO military forces depart.
The Taliban fired rockets into Kabul international airport on Thursday, destroying President Hamid Karzai's helicopter and damaging three other choppers in an attack that underlined the insurgent threat.
"We will not accept the preliminary results that are to be announced Monday because the fake and clean votes have not been separated," Abdullah's spokesman Baryalai Arsalayee told reporters.
"The process is still not transparent, and massive fraud has not been addressed."
Abdullah, once a front-runner in the race, alleges he was the victim of "industrial-scale" ballot-box stuffing, with many more votes than voters registered in some areas.
Ghani claims to have won fairly by more than one million votes and accuses Abdullah of refusing to accept defeat - raising the prospect of a messy election aftermath.
European Union observers on Thursday voiced growing international concern over fraud and called for an audit of suspicious votes to be expanded from 2000 to 6000 polling stations - about a quarter of all ballot boxes.
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