Taliban-led violence has intensified throughout the country with the advent of summer and as thousands of US reinforcements recently have joined the fight against the Islamist militants. Thousands of troops from the US, Britain and Canada are taking part in major anti-Taliban assaults in southern and eastern Afghanistan to provide a safer environment for the upcoming election.

About 40 candidates, including Karzai and two women, are standing in the race.

Although Karzai is widely seen to be the front-runner, intensified violence in the country's southern and eastern regions, the main political power base for the incumbent, is likely to make it difficult for Karzai to secure more than 50 percent of the vote and win the first round.

Sources said Karzai's government and his campaign team have been in contact with some of the Taliban commanders in the south in a bid to persuade the militants not to disrupt the elections in those areas.

Former finance minister Ashraf Ghani Ahmadzai and former foreign minister Abdullah Abdullah are the other leading candidates who could unseat Karzai in the country's second direct vote for president.