All parties to an armed conflict – whether they are States or non-State actors – are bound by the relevant rules of IHL. This is the case even though only States may become party to international treaties, and thus to the four Geneva Conventions and their Additional Protocols.

States Parties must not only ‘respect’ but also ‘ensure respect’ for IHL in all circumstances. While the obligations of non-State armed groups may differ from those of sovereign States, most of the customary rules of IHL apply to all parties to a conflict. Non-State armed groups are generally bound by the treaty IHL rules applicable in non-international armed conflicts as well.

Furthermore, States not party to an armed conflict are required neither to encourage a party to violate IHL nor to take such action as would assist in the commission of violations of IHL. This obligation is generally interpreted as requiring States not party to an armed conflict to take all appropriate measures to prevent or end violations of IHL committed by any party to the conflict.

Source: IHL Guide