Annex to the Convention: Regulations respecting the laws and customs of war on land - Section I : On belligerents - Chapter II : Prisoners of war - Regulations: Art. 5.
Art. 5. Prisoners of war may be interned in a town, fortress, camp, or other place, and bound not to go beyond certain fixed limits; but they cannot be confined except as in indispensable measure of safety and only while the circumstances which necessitate the measure continue to exist.