KABUL - An anti-corruption campaigner who came third in Afghanistan's election said Thursday he may urge a boycott of a run-off next month, as the UN strived to avoid a repeat of the first round's rampant fraud.

The scale of the corruption during the August 20 election, most of which benefited President Hamid Karzai, has led to deep disillusionment in a country beset by a bloody Taliban insurgency.

US Defense Secretary Robert Gates said the run-off on November 7 would be watched closely for fraud and foreign troops would work to protect voters from Taliban attack.

But outspoken lawmaker Ramazan Bashardost, who came a distant third in August with about 10 percent of the vote, damned the entire process as a "failure".

"Elsewhere in the world people involved in fraud are sent to jail but here in our country they are praised," he told AFP.

Bashardost said he would announce early next month whether he is asking his followers to back Karzai or former foreign minister Abdullah Abdullah, or to boycott the new ballot.

The two contenders had already lobbied for his support, he said, as distribution got under way of ballot papers and indelible ink to polling stations across the country.

"But mark my words, I won't trade the vote of Afghan people or my supporters in return for a position" in government, he said. "I'll do what is best for the people of Afghanistan."

One analyst cast doubt on whether Bashardost, a member of Afghanistan's minority Hazara community, had the political clout to make a difference.

"I don't think he's in a kingmaker position," said Haroun Mir, of Afganistan's Centre for Research and Policy Studies.

"The people who voted for him were the ones who are fed up with both Karzai and Abdullah and were the ones who were not affiliated to a political party or tribal group -- that's why they voted for him."

But a boycott call could undermine the credibility of a hastily-arranged poll that comes after weeks of political paralysis, threatening to depress August's low turnout rate of 38 percent still further.

Afghanistan's Independent Election Commission said the vote run-up would begin in earnest on Saturday with the start of a 12-day campaign period until November 5 and the first distribution of electoral materials nationwide.

UN chief Ban Ki-moon has said the poll faces huge challenges and has ordered around 200 monitors who oversaw the first round to be replaced for the run-off.

UN spokesman Dan McNorton said some polling stations where major ballot-stuffing was detected were unlikely to reopen.

"There will definitely be fewer polling stations" in areas "where it is not possible for Afghan security forces to be present to ensure safe voting" and where massive fraud was detected in the first round, McNorton said.

Abdullah's camp is also sceptical about the composition of the electoral commission, a nominally independent body that is seen by many as pro-Karzai.

"If the composition of the leadership of the commission is not changed, no doubt we will still have the crisis of fraud and illegal rule in the country," said a pro-Abdullah political party, the New Way League.

Organisers face a race against time to stage the second round with the onset of winter, as US President Barack Obama considers whether to deploy thousands more troops into the fray.

Asked about security problems surrounding the run-off, Gates said almost all countries that increased troop levels before the August polls had kept those additional forces in Afghanistan.

"Everybody has an interest in making sure that there are as few problems with this run-off election as possible in terms of providing legitimacy for the winning candidate," he said.

NATO chief Anders Fogh Rasmussen urged member nations and partners to step up efforts to build Afghan security forces, warning that failure would destabilise the region and ultimately export that insecurity to Europe.

"Leaving Afghanistan behind would once again turn the country into a training ground for Al-Qaeda," he added.

"The pressure on nuclear-armed Pakistan would be tremendous, insecurity would spread throughout Central Asia and it would only be a matter of time until we in Europe would feel the consequences."