Related Rule
United States of America
Practice Relating to Rule 35. Hospital and Safety Zones and Neutralized Zones
The US Field Manual (1956) restates Article 23 of the 1949 Geneva Convention I and Articles 14–15 of the 1949 Geneva Convention IV and specifies that these agreements setting up hospital and safety zones and neutralized zones “may be concluded either by the governments concerned or by subordinate military commanders”. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, §§ 224 and 253.
The US Air Force Pamphlet (1976) states:
The Geneva Conventions of 1949 provide for protected or safety zones established by agreement between the parties to the conflict. Safety zones established under the Geneva Conventions of 1949, or by other agreement among parties to a conflict, are immune from bombardment in accordance with the terms of the agreement. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-5(b).
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 US Naval Handbook (1995) states: “When established by agreement between the belligerents, hospital zones and neutralized zones are immune from bombardment in accordance with the terms of the agreement concerned.” 
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), § 8.5.1.5; see also § 8.6.2.2 (protected places and objects).
The US Naval Handbook (2007) states: “When established by agreement between the belligerents, hospital zones and neutralized zones are immune from bombardment in accordance with the terms of the agreement concerned.” 
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, § 8.9.1.5.
The Handbook also states: “The [1949] Geneva Conventions recognize the special status of the ICRC and have assigned specific tasks for it to perform, including … offering its ‘good offices’ to facilitate the establishment of hospital and safety zones.” 
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, § 6.2.2.