Both candidates vying to be the next president of Afghanistan are convinced they will win and that only cheating can stop them - setting the stage for a fraught election when campaigning starts Thursday.

Abdullah Abdullah and Ashraf Ghani are locked in a head-to-head battle that could test Afghan stability as the country chooses its first new leader since the tumultuous days after the Taliban regime fell in 2001.

As the drawn-out election process builds to a climax, US-led combat troops are closing bases and withdrawing rapidly, with all remaining 51,000 NATO soldiers due to exit this year after more than a decade fighting the Taliban.

The first-round election on April 5 was hailed a success by Afghan officials and foreign allies after the insurgents failed to launch a major attack, and fraud - though widespread - was deemed not to have affected the outcome.

Abdullah and Ghani, the two top vote-winners, progressed through to a run-off vote on June 14, with some observers fearful that the campaign could be antagonistic and that the final result may be disputed.

"The worst-case scenario would be if the election is both polarising and the results are not accepted by one of the candidates - that has the potential to lead to conflict," Nicholas Haysom, deputy UN mission chief, said this week.

"The remedy to that is to make the election as transparent as possible."

Many Afghans fear polling day will be targeted by the Taliban as the date falls at the height of the traditional "fighting season", presenting the insurgents with a timely moment to grab back the headlines.

"A second round gives an opportunity for a much more effective and complete democratic exercise to choose a leader," Haysom said. "But it comes at a risk."

He added that the UN had appealed to both candidates not to campaign "in a manner which promotes the risk of ethnic tensions and violence".

Playing the ethnic card?

Ethnic divisions have blurred in Afghanistan to some extent since 2001, but tribal affiliation remains a key factor in voting preferences.

Abdullah attracts much of his support from the Tajik ethnic minority and other Dari-speaking groups in the north, while Ghani is a Pashtun, the largest ethnic group, which is strongest in the south and east.

President Hamid Karzai, who must step down after serving the maximum two terms, retained power in 2009 in a fraud-riddled election that nearly derailed the multinational effort to develop Afghanistan.

A close election next month could revive the tensions and bitterness of 2009, when Abdullah dramatically pulled out of the run-off after massive cheating by Karzai's supporters.

"If the second-round is tainted by fraud, it would be extremely dangerous for a country like Afghanistan," said European Union election observer Thijs Berman, who described fraud in the first round as "substantial". "Any fraud is a threat to the security of Afghanistan and its future. It is essential that there is a peaceful transition of power."

Berman criticised Afghan election authorities for releasing "late and incomplete" data after the first round, and he said better transparency was essential in the run-off.

Since the second-round timetable was declared last week, Ghani has been tweeting about new allies joining his team while Abdullah has spoken of his priorities when he moves into the presidential palace.

With neither contender likely to concede without a fight, the election process that began in early February could hold more surprises before final results of the run-off are declared on July 22. Talk of a power-sharing deal to avoid another election has faded, with the race now seen as likely to go down to the wire.

The Free and Fair Election Forum of Afghanistan (FEFA) monitoring group warned election officials that they would be under more pressure than ever. "Supporters of candidates, regional stakeholders and even election officials were involved in fraud in the first round," FEFA spokesman Fahim Naeemi told AFP. "We urge the electoral commissions to take serious measures to address the shortcomings."–AFP