Taliban militants in Afghanistan warned Saturday that they will ramp up attacks against civilians during next month's presidential runoff, in an attempt to keep Afghans away from polling centres on election day.

The Taliban issued its warning on the first official day of campaigning for the vote, which was scheduled for Nov. 7 after the August election failed to produce a clear winner.

While incumbent Hamid Karzai led the initial vote count with about 54 per cent of ballots cast, the UN-back Electoral Complaints Commission threw out a third of his votes due to widespread fraud.

In its warning, the Taliban called the elections "a failed, American process" and vowed to "launch operations against the enemy and stop people from taking part."

According to the statement, militants will blockade roads and highways and said those who choose to vote "will bear responsibility for their actions."

Taliban violence during the election's first round on Aug. 20 killed dozens of civilians.

Militants hammered a number of cities, particularly in the south, with rocket fire and cut off the ink-stained fingers of at least two voters. In Afghanistan, fraud-prevention measures include dipping voters' fingers in blue ink so they can only cast one ballot.

While the Afghan Independent Election Commission must worry about security in the few weeks it has to prepare for another election, there are also concerns that weather will keep voters away from polling centres.

Winter settles in by the middle of November in Afghanistan, making roads impassable with snow and cutting off isolated villages.

Meanwhile, as the second round of campaigning got underway, officials working with Karzai challenger Abdullah Abdullah called for three members of the election commission to step down on accusations of bias.

Commission spokesperson Noor Mohammad Noor, denied there was bias on the panel and said it would be "impossible" to replace the members.

The commission, filled with Karzai supporters, must ensure the runoff is free from fraud allegations, which discredited the Afghan government in the eyes of locals and among the international community.

The runoff comes at a time when U.S. President Barack Obama is considering a request to send tens of thousands more troops to Afghanistan. Officials in the Obama administration have said a stable, credible government in is a prerequisite for a ramped up military campaign against a growing militant insurgency.

To help prevent cheating, election officials have cut about 7,000 of the 24,000 polling stations that were established for the August vote. Some of those polling stations never opened, allowing officials to stuff ballot boxes.

With files from The Associated Press