Related Rule
United States of America
Practice Relating to Rule 36. Demilitarized Zones
Section A. Establishment of demilitarized zones
The US Air Force Pamphlet (1976) states: “Both the 1923 Draft Hague Rules [of Air Warfare] and the 1949 Geneva Conventions recognize the right of states, by agreement, to create safety zones or demilitarized zones.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-4(c).
The US Air Force Commander’s Handbook (1980), in a section entitled “Neutralized and Demilitarized Zones”, provides:
By agreement, the parties to a conflict may establish certain zones where civilians, the sick and wounded, or other noncombatants may gather to be safe from attack.
A party to conflict cannot establish such a zone by itself; neutralized zones need only be respected if established by agreement between the parties, either oral or written, or by parallel declarations. Such an agreement may be concluded either before or during hostilities.
United States forces need not respect such a zone unless the United States has agreed to respect it. Even in an unrecognized zone, of course, only legitimate military objectives … may be attacked. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-6(b).
The Report on US Practice considers that US opinio juris generally conforms to the rules and conditions prescribed in Article 60 of the 1977 Additional Protocol I. 
Report on US Practice, 1997, Chapter 1.8.