Related Rule
United States of America
Practice Relating to Rule 36. Demilitarized Zones
Section B. Attacks on demilitarized zones
The US Air Force Pamphlet (1976) states:
Doubtless the creation of [safety or demilitarized] zones would be one of the most effective measures to enhance protection of one’s own civilian population, and if the conditions required to make a zone were fulfilled and maintained, virtually all civilian casualties would be avoided in this zone. 
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 in 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 US Naval Handbook (1995) provides: “An agreed demilitarized zone is also exempt from bombardment.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 8.5.1.3; see also § 8.6.2.2 (protected places and objects).
The US Naval Handbook (2007) states: “An agreed demilitarized zone is also exempt from bombardment.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 8.9.1.3.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed: “We support the principle that attacks shall not be made against appropriately declared or agreed demilitarized zones.” 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 427.
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.