The U.S. State Department Thursday designated as terrorists three leaders of the Nigeria-based Islamist militant group Boko Haram. The State Department said in a news release Boko Haram, which operates mainly in northern Nigeria, and associated militants have killed more than 1,000 people in the past 18 months. Abubakar Shekau is the most visible of the three members designated as terrorists, the department said. The two others, Abubakar Adam Kambar and Khalid al-Barnawi, have close links to al-Qaida in the Islamic Maghreb, which has been designated a terrorist organization by the United States. Boko Haram, whose name means \"Western education is forbidden,\" claimed responsibility for the Christmas Day 2011 attack on St. Theresa Catholic Church in Madalla, Nigeria, which killed at least 35 people and injured dozens of others. The State Department said a series of Boko Haram attacks on Jan. 20, 2012, in Kano had killed more than 180 people. The overwhelming majority of the group\'s victims have been civilians, the department said. The State Department designations freeze any assets belonging to the men in the United States and forbids contact with U.S. citizens, the BBC said. \"These designations demonstrate the United States\' resolve in diminishing the capacity of Boko Haram to execute violent attacks,\" the department said in the release. the