Three troops in the NATO-led force in Afghanistan have been killed in an attack by a member of the Afghan security forces, a US defense official said Wednesday, in the latest \"insider attack\" that has caused growing dismay among coalition commanders. The assault occurred in the evening hours in Oruzgan province and the troops killed were not from the United States but their nationality had yet to be announced, said the US defense official, who spoke on condition of anonymity. About 1,500 Australian troops are deployed in the province as part of the International Security Assistance Force (ISAF).