Taliban militants ambushed a truck carrying police in southern Afghanistan, killing four officers and injuring three at the site of NATO's largest-ever ground battle, while a separate gunfight left 11 Taliban fighters dead, officials said Saturday. The attack on the police occurred Friday evening in Kandahar province's Panjwayi district, said provincial police chief Asmatullah Alizai. The three injured were in serious condition, he said. NATO forces killed more than 500 militants in the Panjwayi area in September, largely clearing the region of militants in an offensive called Operation Medusa. Canadian and US Special Forces are working to increase security in Panjwayi. More Afghan security checkpoints have been set up, a new road is being built, and schools and clinics are reopening. Yet small groups of fighters remain.