From HSToday:

Amid rising fears of the potential for a terrorist attack on the US homeland, the Department of Homeland Security (DHS) today announced the addition of Libya, Somalia, and Yemen to the list of countries ineligible to use the Visa Waiver Program (VWP) for travel to the United States. Last month, the United States began implementing changes under the Visa Waiver Program Improvement and Terrorist Travel Prevention Act of 2015. Travelers from those regions labeled “countries of concern” can no longer travel to the US under the VWP.