The United States blacklisted two top leaders of Somalia's Shebab on Tuesday, a day after the Islamist group killed seven people, including four UN workers, in a huge bus bombing.

Ahmed Diriye, who took over as the militant group's leader in September 2014, and intelligence chief Mahad Karate, were both designated as terrorists by the State Department, which said the group was behind several atrocities, including the 2013 Westgate Mall attack in Nairobi and the April 2 massacre at Garissa University in Kenya in which 148 people were killed.