In general, U.S. citizens and G.C. holders are required to pay U.S. federal income tax whether or not they live in the U.S. This is the position taken by the U.N.
However, the IRS website states that two organizations in the world are exempt from this rule: the IMF and the World Bank.
Your work for an international organization and the international organization agreement creating the international organization provides that alien employees are exempt from U.S. income tax. Two international organizations that have such a provision are the International Monetary Fund (IMF) and the International Bank for Reconstruction and Development (World Bank).