The Taliban meanwhile took credit for the blast the day before outside the huge tent where next week's Loya Jirga is to be held later this week.
Elsewhere, in the insurgency-racked south, villagers discovered the beheaded bodies of six government contractors and a service member from the international coalition died when his vehicle hit a roadside bomb, officials and NATO said.
Yesterday's suicide bomber detonated his explosive-laden vehicle after being spotted by Afghan security personnel guarding the Loya Jirga site, said Interior Ministry spokesman Sediq Sediqqi. He said three of the 12 dead were members of Afghanistan's National Security Force with most of the rest civilians.
Hours before, President Hamid Karzai announced that U.S. and Afghan negotiators had completed a final draft of the Bilateral Security Agreement to be presented to the gathering for debate. If approved, it would allow U.S. Troops to stay in Afghanistan after the final withdrawal of international combat forces in 2014.
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The Taliban took responsibility for the bombing, giving the bomber's name as Saeed Kabuli. They provided few other details.
So far this year 139 NATO service members have been killed in Afghanistan, according to Associated Press figures. This compares to 394 troops of the NATO-led coalition killed in 2012.
Also in southern Afghanistan, where a stubborn insurgency flourishes, police said they recovered the decapitated bodies of six government contractors, the apparent victims of Taliban insurgents who regularly target state projects.
Kandahar police spokesman Ahmed Durrani said villagers found the bodies today. He said the men were involved in building police compounds and checkpoints in Kandahar, the birthplace of the Taliban.
There was no immediate claim of responsibility for either attack, but the Taliban have previously targeted both officials and contractors, warning Afghans against working for the government.