Former US Secretary of the Treasury and Secretary of State George Shultz, former U.S. Secretary of Defense William Perry and former U.S. Senator Sam Nunn on Monday called on Singapore's Prime Minister Lee Hsien Loong on nuclear threat and other issues of common concern. According to a statement by the city-state's Ministry of Foreign Affairs, the prime minister and the U.S. politicians exchanged views on regional and international developments during the meeting. The topics included the work of the Nuclear Threat Initiative, a non-profit, non-partisan organization working to reduce threats from nuclear, biological and chemical weapons. The three U.S. statesmen are here to attend a Nuclear Threat Initiative meeting which will be held from Tuesday to Wednesday.