Related Rule
United States of America
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
Section B. Establishment of non-defended localities
The US Air Force Pamphlet (1976) states:
A party to a conflict may declare, as undefended, inhabited localities which are near or in areas where land forces are in contact when the localities are open for occupation by an adverse party. Bombardment in such a locality would be unlawful, if the following conditions were met and maintained: (1) no armed forces or other combatants present, (2) no mobile weapons or mobile military equipment present, (2) no hostile use of fixed military establishments or installations, (4) no acts of warfare by the authorities or the population, and (5) no activities in support of military operations. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-3(e).