Gunmen have killed two aides of an Afghan presidential candidate, hours before the official election campaign was due to start. The attack, which took place on Saturday, was confirmed by a spokesman for the candidate, Abdullah Abdullah. Taliban fighters have threatened to target the campaign, and the Afghan police and army face a major challenge as the US-led NATO troops prepare to exit. No one has yet claimed responsibility for the attack. Al Jazeera's Jane Ferguson reports from Kabul.