From the United States Department of State:  "Travel Warnings are issued to describe long-term, protracted conditions that make a country dangerous or unstable. A Travel Warning is also issued when the U.S. Government's ability to assist American citizens is constrained due to the closure of an embassy or consulate or because of a drawdown of its staff. The countries listed below meet those criteria." 
See the US Department of State's site for up to date information on travel warnings by country.