This article examines the law governing siege warfare and its application to sieges enforced by parties to the Syrian conflict. In doing so, the article considers the classification of the conflict and questions whether the conflict has crystallized into an international armed conflict. It critically applies the laws of armed conflict to the ongoing sieges and examines the obligations of parties to the conflict in relation to humanitarian assistance and evacuation of civilians in sieged areas. In the event that humanitarian assistance is denied, this article explores the criminalization of the resulting starvation of civilians as war crimes, crimes against humanity and genocide. There is compelling evidence to conclude that the war crime of starvation and the crime against humanity of extermination have been committed during siege warfare. These crimes ideally should be prosecuted by international judges through the establishment of an ad hoc international criminal tribunal or immediately through universal jurisdiction.