This article argues that since the Tadić case before the International Criminal Tribunal for the Former Yugoslavia, a new category of armed conflict has migrated from international criminal law to international humanitarian law: that of armed groups fighting each other within the borders of a state without the intervention of the armed forces of the latter. However, the extent to which the rules of this category of conflict cover issues that may arise in such a conflict has not been comprehensively examined. One may infer, from the war crimes that the Rome Statute of the International Criminal Court criminalizes in this type of conflict, a dozen rules of international humanitarian law. After giving an historical account of the codification of this category of armed conflict, the author argues that there is a need to further develop these rules in order to provide a more comprehensive humanitarian law regime applicable to conflict exclusively between non-state armed groups. The absence of such a comprehensive regime should not, however, be taken as evidence of a legal vacuum. The author suggests that a law enforcement regime resting on international human rights law should be applied to relations between the armed groups and the territorial state, while the warring relationship between the armed groups should fall under the law of armed conflict, including those core customary rules that are now recognized as being equally applicable to international and non-international armed conflict.