REPUBLIKA.CO.ID, KABUL -- Afghanistan's election campaign kicks off Sunday as the killing of a presidential candidate's aides highlighted the threat surrounding the poll to succeed Hamid Karzai, with NATO combat troops due to withdraw by year end.

Gunmen shot dead two aides of 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.

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.

Karzai has ruled the country since the fall of the Taliban 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.

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