International humanitarian law is very clear that occupation must only be temporary; the Israeli settlements are in direct violation of this principle. For instance, the settlements are in breach of Article 49 of the 4th Geneva Convention , which forbids an occupier from transferring its own civilians into the territory it occupies. Additionally, according to Article 55 of the Hague Regulations , the occupying power’s role is to safeguard occupied properties and maintain the status quo.
As the international development organization Diakonia explains , the construction of settlements violates this article because of the major changes it inflicts upon the occupied territory. And, according to Article 43 of the Hague Regulations , the occupying power must uphold order and safety while respecting the laws of the occupied country. Yet as Diakonia also explains , the settlements actually undermine public order and violate existing laws.
See also the following pages from the B’tselem (The Israeli Information Center for Human Rights in the Occupied Territories) website:
- Land Expropriation and Settlements in the International Law 
- Land Expropriation and Settlements