KABUL (AFP) - Afghanistan's election campaign kicks off on Sunday as the killing of a presidential candidate's aides highlighted the threat surrounding the poll to succeed Mr Hamid Karzai, with Nato combat troops due to withdraw by year end.

Gunmen shot dead two aides of Mr Abdullah Abdullah, a former foreign minister seen as a strong contender, in the western city of Herat on Saturday, officials said.

The attack comes as the country prepares for its first democratic transfer of power, with the April 5 election viewed as a key test of the effectiveness of the 350,000-strong Afghan security force as foreign troops prepare to exit the country.

A dispute between Kabul and Washington over whether a small force of US soldiers stays behind beyond 2014 is likely to dominate the campaign.

Mr Karzai was expected to sign a bilateral security agreement (BSA) late last year, which would allow about 10,000 US troops to be deployed in the country after NATO withdraws by December.

But he has stalled and said his successor might now complete negotiations - plunging relations with the US, Afghanistan's key donor, to a fresh low.

Mr Karzai has ruled the country since the fall of the Taleban in 2001, surviving assassination attempts and the treacherous currents of Afghan political life as billions of dollars of military and development aid poured into the country.

He is barred from seeking a third term, leaving an open field to compete in the April 5 vote, which is likely to trigger a second-round run-off in late May between the two strongest candidates.

Tipped to go through to the run-off stage is Abdullah, the suave opposition leader who came second to Karzai in the chaotic and fraud-riddled 2009 election.

Among the other heavyweight candidates are former finance minister Ashraf Ghani, Karzai loyalist Zalmai Rassoul and the president's low-profile elder brother Qayum Karzai.

In comments likely to cause further friction with his Nato allies, Mr Karzai criticised their conduct of the 12-year conflict in an interview with Britain's Sunday Times in which he described the Taleban as "brothers" and the US as "rivals".

Mr Karzai told the Times "the US-led Nato mission in terms of bringing security has not been successful, particularly in Helmand", a southern stronghold of Taleban militants.

"We have immense respect for the life of Nato soldiers lost in Afghanistan and strong disagreement for the way US conducted itself in Afghanistan," he said.

Western and Afghan officials say all 11 candidates support the BSA but, except for Abdullah, they have declined to say so publicly for fear of clashing with Mr Karzai.

Taleban insurgents have threatened to target the campaign, and the Afghan police and army face a major challenge with little support from the dwindling number of Nato troops.

The interior ministry hopes to open 6,431 of the 6,845 polling centres, though fear of insurgent violence could keep turnout low.

Only about one-third of registered voters cast their ballots last time - significantly lower than previous elections - and the turnout may decline further.

Debate on policy and economics is set to take a back seat to ethnic and tribal loyalties, with runners picking vice-presidential candidates to widen their appeal.

The first visible sign of the election is likely to be candidates' posters plastered on walls across the country, with relatively little action on the campaign trail until nearer the vote.

Television debates and large open-air rallies are also planned, but no details have been confirmed.

Disputes over millions of fraudulent ballots led to a major crisis after voting in 2009, before Mr Abdullah pulled out of the run-off, leaving Mr Karzai to take power.

Election organisers are again expected to be busy with complaints of fake votes, ballot-box stuffing and polling booths unable to open due to intimidation.

"Holding elections is not an easy job in the current situation in Afghanistan," Yousuf Nuristani, chairman of the Independent Election Commission, told candidates recently.

"We hope you carry out your election campaigns in accordance to the law and in a good environment."

The next president is likely to be inaugurated in July or August.