The Afghan Taliban threatened on Tuesday to behead U.S. troops in revenge for the massacre of 16 Afghan civilians by a lone American soldier in southern Afghanistan, killings that sparked condemnation and Afghan calls for an early U.S. withdrawal. ''The Islamic Emirate once again warns the American animals that the Mujahideen will avenge them, and with the help of Allah will kill and behead your sadistic murderous soldiers,'' Taliban spokesman Zabihullah Mujahid said in an emailed statement, using the term with which the Islamist group describes itself. A U.S. Army staff sergeant has been taken into custody, accused of walking off his base in the volatile province of Kandahar in the middle of night and gunning down at least 16 villagers, mostly women and children.