The Hague Convention of 1907 states;
"Article 25: The attack or bombardment, by whatever means, of towns, villages, dwellings, or buildings which are undefended is prohibited.
Article 26: The officer in command of an attacking force must, before commencing a bombardment, except in cases of assault, do all in his power to warn the authorities.
Article 27: In sieges and bombardments all necessary steps must be taken to spare, as far as possible, buildings dedicated to religion, art, science, or charitable purposes, historic monuments, hospitals, and places where the sick and wounded are collected, provided they are not being used at the time for military purposes.
It is the duty of the besieged to indicate the presence of such buildings or places by distinctive and visible signs, which shall be notified to the enemy beforehand"