KABUL, Aug 13 (Reuters) - The Taliban have captured Afghanistan"s second biggest city of Kandahar, officials said on Friday, in the most significant setback for the U.S.-backed government since the insurgents launched a new offensive as U.S. forces withdraw. "Following heavy clashes late last night the Taliban took control of Kandahar city," a local government official told Reuters after the militants announced they had taken it. Government forces were still in control of Kandahar"s airport, which was the U.S. military"s second biggest base in Afghanistan during their 20-year mission.