International Committee of the Red Cross (ICRC) 05-10-2010 FAQ
When a territory is placed under the authority of a hostile army, the rules of international humanitarian law dealing with occupation apply. Occupation confers certain rights and obligations on the occupying power.
Prohibited actions include forcibly transferring protected persons from the occupied territories to the territory of the occupying power.
It is unlawful under the Fourth Geneva Convention [Article 49] for an occupying power to transfer parts of its own population into the territory it occupies. This means that international humanitarian law prohibits the establishment of settlements, as these are a form of population transfer into occupied territory. Any measure designed to expand or consolidate settlements is also illegal. Confiscation of land to build or expand settlements is similarly prohibited.