At least nine Afghan aid workers were killed in an armed attack in Afghanistan's Balkh province, an official said on Tuesday."Unknown militants attacked the camp of the aid workers in a village in Zari district overnight and fled after gunning down nine aid workers," the official told Xinhua. One woman was among those killed.The victims were in charge of implementing a project under the National Solidarity Programme, said Abdul Basid Hainy, head of the provincial rural development and rehabilitation department.