KABUL (AFP) - Afghanistan's rival presidential candidates clashed over alleged fraud on Sunday ahead of the release of preliminary election results, setting the stage for a prolonged squabble that threatens worsening instability.

Mr Abdullah Abdullah demanded that the preliminary result, due out on Monday, be delayed for a comprehensive audit of suspicious votes, repeating that he would reject the outcome if his demands were not met.

But Mr Ashraf Ghani, who is reported to be at least one million votes ahead in the count, insisted that the result could not be delayed again and must be revealed by the Independent Election Committee.

The preliminary result will include all votes from the June 14 run-off vote, with the official result scheduled for July 24 after a period for adjudication of complaints.

The dispute over fraud has tipped Afghanistan's first democratic transfer of power into turmoil as United States-led troops end their 13-year war against the Taleban, and the country faces a new era with declining civilian aid.

"What we are asking for is thorough auditing, then preliminary results afterwards," Mr Abdullah told supporters in Kabul. "We are honoured to claim that we are the majority of votes in Afghanistan that are clean.

"I will accept the result when the clean votes are separated are from unclean votes."

Mr Abdullah's stance came despite reports that the two candidates were close to a deal agreeing that the audit would be held, but only after the preliminary results.

Discussions between the two campaigns were set to continue into the night.

Influential US Senator Carl Levin, on a visit to Kabul, told reporters that he expected a compromise to be reached.

"Tomorrow there's going to be an announcement of the result," Sen Levin, chairman of the Senate committee on armed services, said.

"Something else is going to be announced... I expect that something else is going to be an agreement on a comprehensive audit.

"We have assurances they will accept the outcome of such an audit."

Mr Ghani's team earlier said that no further delays in the result were unacceptable.

"This is a red line for us," Ms Azita Rafhat, a Ghani spokesman, told AFP. "We gave them enough time, and now the people want to know the result of their votes."

The US and Afghanistan's other international backers had pushed hard for a smooth election as Nato troops pull out, but the fraud allegations have provoked a major political crisis in Kabul.

Any prolonged power struggle would undermine claims that the costly US-led military and civilian mission has helped to establish a functioning state in Afghanistan.

It could also threaten billions of dollars of aid pledges, and boost the insurgents.

The government on Sunday rejected a proposal to ban Facebook during the election deadlock, despite fears that social media postings have fanned ethnic hatred.

Mr Ghani attracts much of his support from the Pashtun tribes of the south and east, while Mr Abdullah's loyalists are Tajiks and other northern Afghan groups - echoing the ethnic divisions of the bloody 1992-1996 civil war.

The United Nations mission in Afghanistan has highlighted the risk of political tensions spilling over into civil unrest, though protests have so far been peaceful.

President Hamid Karzai, who has ruled since the fall of the Taliban in 2001, is constitutionally barred from a third term in office, and he has not publicly endorsed one candidate.

With Nato's combat mission ending, the summer has been a major test of the fledging Afghan government forces now responsible for imposing nationwide security.

All foreign combat troops will leave Afghanistan by December, with about 10,000 US troops staying into next year if the new president signs a security deal with Washington.