Over the past decades, there has been a significant increase in the number of armed groups involved in armed conflicts around the world, as well as in their impact on the rights and freedoms of the population under their control. Facing various situations of systematic violations of human rights by non-state actors, experts in the field of international human rights law began to consider the theoretical justification for the mandatory nature of the provisions on the observance and protection of human rights for armed groups. In this regard, a number of scholars have turned to the theory of customary international law, the acceptability of which is being investigated by the author of this paper. The author examines the provisions underlying this theory and the persuasiveness of the argumentation used by its supporters. Based on an analysis of the nature of customary international law, its structural elements, their interpretation by the UN International Court of Justice in its decisions and the relationship of customary international law with peremptory norms of jus cogens, the author concludes that the theory under consideration is currently unable to explain the existence of obligations of armed groups in the field of human rights.