KABUL - The top U.N. official in Afghanistan warned Tuesday that next month's presidential elections will be the most complicated he has ever seen, while a gunman opened fire on a campaign team for the top challenger to President Hamid Karzai, killing one campaign worker.

Some 17 million registered voters are eligible to vote Aug. 20 for Afghanistan's next president and provincial council members - but the logistics of setting up polling centres in a country plagued by widening insurgency, rugged terrain and a lack of infrastructure has proved challenging.

In an example of the danger the campaigns face, gunmen opened fire on a campaign manager for former Foreign Minister Abdullah Abdullah in eastern Afghanistan. The campaign manager was wounded and a driver was killed, said Laghman province's deputy governor. Abdullah is believed to be Karzai's top challenger.

"These are the most complicated elections I have seen," Kai Eide, the top U.N. official in Afghanistan, said while surveying a cavernous hangar in Kabul where election materials packed and sealed in blue plastic boxes were being loaded onto trucks for delivery to the provinces.

Eide said donkeys will be used to carry ballots to the country's most inaccessible regions, areas that trucks and even helicopters cannot reach. Some 3,171 burros will be loaded with ballots and voting boxes and sent along the steep ridges of the Hindu Kush mountain range, which bisects the middle of Afghanistan.

Another concern is how to hold polls in the turbulent south and east, where U.S. and British forces fighting the Taliban this month have suffered their highest casualties of the eight-year war.

Of 7,000 polling centres planned across the country, security forces have not yet confirmed whether voting can take place in about 700 of them, said Noor Mohammad Noor, a spokesman for the Independent Electoral Commission.

Western officials say that legitimacy of the elections - the third national polls since the Taliban were ousted from power in 2001 - may hinge on the ability to hold them in as many areas of the south and east as possible.

Almost all the problematic polling centres are in the country's ethnic Pashtun areas, where the insurgency is the strongest, said a Western official working on the elections who spoke on condition of anonymity because he was not authorized to speak publicly.

The inability of the authorities to secure hundreds of polling centres, coupled with a potential low turnout among voters in provinces mired in war could threaten the legitimacy of the poll in the eyes of many Afghans, the official said.

Karzai is considered the favourite to win a second five-year term but his chances could hinge on the turnout by his fellow Pashtuns, the country's biggest ethnic group and the heart of the Taliban ranks.

Karzai's only serious competition in the 39-candidate field is believed to be Abdullah, who could force a runoff if low turnout among the Pashtuns prevents Karzai from claiming a majority of the votes.

In an interview Monday with The Associated Press, Karzai reached out to disaffected Pashtuns, calling for a dialogue with Taliban members who are not affiliated with al-Qaida and who are willing to repudiate violence.

Eide said is "tremendously important" to provide access to the polling stations to as many people as possible.

"The aim is to make as much of the country as possible secure for the elections to take place," Eide said.

The attack on Abdullah's campaign was the second fatal assault on his team this month. A local campaign manager was killed in Kapisa earlier this month. Tuesday's attack was carried out by unknown insurgents, said Laghman deputy governor Enayutullah Qalanderzia.

Separately, a roadside bomb in southern Helmand province killed eight Afghan private security guards in Gereshk district on Tuesday, the Interior Ministry said in a statement. Four others were wounded, the statement said.