The United Nations (UN) has a significant role in strengthening nuclear security worldwide. UN Security Council resolution (UNSCR) 1540 and the International Convention on the Supression of Acts of Nuclear Terrorism (ICSANT) constitute key elements of the international legal framework for states to combat nuclear terrorism. The UN General Assembly and Security Council, particularly through its resolutions, can raise awareness and promote cooperation.
In 2004, the United States co-sponsored UNSCR 1540, which requires all States to take meausures to account for, secure, and physically protect nuclear materials in production, use, storage and transport, among other nonproliferation obligations. The United States meets its UNSCR 1540 obligations and assists other States in meeting theirs.
Through the General Assembly, the United States supported the development of the ICSANT. This treaty, which the United States ratified in 2015, creates an international legal framework to combat nuclear terrorism, including measures that criminalize nuclear terrorism and steps to strengthen cooperation between States on nuclear terrorism. The United States also cooperates closely with the UN Office of Disarmament Affairs, the UN Office of Drugs and Crime, and other UN Secretariat bodies in enhancing nuclear security worldwide.
Barack Obama, Fact Sheet: U.S. Participation at the United Nations on Nuclear Security Issues Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/318245