The United States Army Special Forces, also known as the Green Berets because of their distinctive service headgear, are a special operations force. Army Special Forces are tasked with six primary missions: unconventional warfare, foreign internal defence, special reconnaissance, direct action, hostage rescue, and counter-terrorism. The first two emphasize language, cultural, and training skills in working with foreign troops. Other duties include combat search and rescue (CSAR), security assistance, peacekeeping, humanitarian assistance, humanitarian demining, counter-proliferation, psychological operations, manhunts, and counter-drug operations; other components of the United States Special Operations Command or other U.S. government activities may also specialise in these secondary areas. Many of their operational techniques are classified, but some nonfiction works and doctrinal manuals are available.