The leading candidates in the race to become Afghanistan's next president have started lobbying in anticipation of a run-off following final preliminary election results this weekend.

None of the eight runners would appear to have won an absolute majority.

Abdullah Abdullah and Ashraf Ghani - both former ministers in president Hamid Karzai's ­government - share three-quarters of the votes counted so far, but voting trends show that neither will secure the 50% needed to win outright.

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The expected Abdullah-Ghani run-off would take place at the end of next month.

Former foreign minister Zalmai Rassoul, who is running a distant third with 11%, and former Islamist warlord Abdul Rasoul Sayyaf, with 7%, are emerging as the kingmakers likely to be heading into the second round.

Access to Rassoul's support base is seen as crucial as he is believed to have the backing of the powerful Karzai family.

Both Ghani and Abdullah say that the outgoing leader will have a place on their team in an advisory role.

"Sayyaf is a wildcard," said Graeme Smith, a Kabul-based analyst with the International Crisis Group.

"He showed surprising strength in the campaign and his voting block is likely to remain more coherent in a second round than the coalition that stood behind Rassoul, which gives Sayyaf some bargaining power as he sits down with the Abdullah and Ghani camps."

After 12 years in power, Karzai is constitutionally barred from running again.

Abdullah has already reached out to Rassoul while Ghani's camp has refused to detail its behind-the-scenes negotiations.

Unless the pair strike a deal to avoid what would be a costly and risky second round of voting, a run-off between them would take place in late May.

Both have said the full democratic process should be completed, a sentiment echoed by the country's Western allies - although some observers have said they can see the merit of a deal that would swiftly move on the political transition.

There are also concerns about the election's security and cost - the first round was funded by Washington to the tune of more than $100 million. With some ballot boxes carried by donkey or mule to and from remote parts of Afghanistan, restarting the entire election process means it could be July before a new president is declared.

The United States has not stated publicly a preference for a candidate, and seem content the top contenders have said they will sign a security agreement that will allow some US troops to stay behind after a December deadline for foreign forces to leave.

US relations with Karzai have deteriorated sharply in the past year. One of the sticking points has been his refusal to sign the bilateral security agreement permitting the US military to stay on to train Afghan forces.

This weekend's final preliminary count will exclude votes being investigated for fraud, which number up to half a million ballots.