A suicide bomber attacked an air force bus in Afghanistan's capital early Wednesday, killing eight military personnel and wounding another 13, security officials said.The Defence Ministry confirmed the number of casualties in a statement updating a previous toll. Army Gen. Kadamshah Shahim said the bomber was stopped before he could enter the bus, likely limiting the number of casualties.The Taliban claimed responsibility for the attack, with a spokesman for the insurgents, Zabihullah Mujahid, confirming that the air force bus was the target.The attack came as the country struggles through its first democratic transition of power, with one of the candidates in last month's presidential runoff alleging fraud. Abdullah Abdullah, a former foreign minister and one of two candidates vying to succeed President Hamid Karzai, has warned he will boycott the results of the June 14 balloting.