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. 15.
Art. 15. Relief societies for prisoners of war, which are properly constituted in accordance with the laws of their country and with the object of serving as the channel for charitable effort shall receive from the belligerents, for themselves and their duly accredited agents every facility for the efficient performance of their humane task within the bounds imposed by military necessities and administrative regulations. Agents of these societies may be admitted to the places of internment for the purpose of distributing relief, as also to the halting places of repatriated prisoners, if furnished with a personal permit by the military authorities, and on giving an undertaking in writing to comply with all measures of order and police which the latter may issue.