What humanitarian laws must be abided by all parties involved in a civil war?

Thanks to the four Geneva Conventions of 1949 and the Additional Protocol of 1977, international humanitarian law is very clear when it comes to international conflicts, which are conflicts between two parties, usually seen as between two separate states. International humanitarian law aims to protect people from crimes against humanity, such as genocide. However, international humanitarian law becomes a bit murky with respect to non-international conflicts, or civil wars.

International humanitarian law was developed to protect the victims of armed conflict such as the "wounded and sick, shipwrecked, prisoners of war and civilians" (Rowe, P., "Freedom Fighters and Rebels: The Rules of Civil War"). However, when it comes to civil wars, the laws of the state and international humanitarian law overlap. Rebels, meaning those in opposition to the state, when captured, are permitted to be prosecuted "under the national law of the state," which means rebels can face charges "ranging from treason, murder and assault to destruction of property" (Rowe). Tribunals in some countries, such as the International Tribunal for the Former Yugoslavia, have established that rebels can be charged with the crimes against humanity and genocide established by the Geneva Conventions; however, currently, no laws can be applied to rebels of every nation.

Our greatest hope lies in the International Criminal Court (ICC) that was established by Roman Statute and signed into ratification by 60 states in 2002. The ICC makes it possible try people of participating states based on international humanitarian laws established by the Geneva Conventions, especially if the country is "unable or unwilling to prosecute their war criminals" themselves (Donovan, D., "International Criminal Court: Successes and Failures"). However, the ICC is running into issues concerning strength of prosecution and, therefore, has not yet accomplished what politicians had hoped to accomplish (Donovan). Therefore, international humanitarian law still remains relatively murky when it comes to the question of civil wars.