Related Rule
United States of America
Practice Relating to Rule 37. Open Towns and 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).
The US Field Manual (1956) reproduces Article 25 of the 1907 Hague Regulations and states:
In addition to the “grave breaches” of the Geneva Conventions of 1949, the following acts are representative of violations of the law of war (“war crimes”):
d. Firing on localities which are undefended and without military significance. 
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, §§ 39 and 504(d).
The US Air Force Pamphlet (1976) reproduces Article 25 of the 1907 Hague Regulations and states:
Cities behind enemy lines and not open to occupation may contain military objectives. The application of this undefended rule to aerial warfare, where the object of the attack was not to occupy the city but to achieve some specific military advantage by destroying a particular military objective, caused disagreements in the past. In the US view, it has been recognized by the practice of nations that any place behind enemy lines is a defended place because it is not open to unopposed occupation. Thus, although such a city is incapable of defending itself against aircraft, nonetheless if it is in enemy held territory and not open to occupation, military objectives in the city can be attacked. 
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).
The US Air Force Commander’s Handbook (1980) states:
Towns, villages, cities, refugee camps, and other areas containing a concentration of civilians should not be bombarded if they are undefended and open to occupation or capture by friendly ground forces in the vicinity. Any military objectives that might exist in these towns (for example, military supplies) can be seized or destroyed by the ground forces. 
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(a).
The US Instructor’s Guide (1985) states:
The attack or shelling by any means whatsoever of undefended towns, villages, dwellings, or buildings is prohibited. This means that military targets can be attacked wherever they are located, but a town with no military targets must be spared. 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 6.
The manual also provides: “In addition to the grave breaches of the Geneva Conventions, the following acts are further examples of war crimes: … firing on facilities which are undefended and without military significance.” 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 13.
The US Rules of Engagement for Operation Desert Storm (1991) prohibits firing at civilian populated areas or buildings which are not defended nor are being used for military purposes. 
United States, Desert Storm – Rules of Engagement, Pocket Card, US Central Command, January 1991, reprinted in Operational Law Handbook, International and Operational Law Department, The Judge Advocate General’s School, United States Army, Charlottesville, Virginia, 1995, pp. 8-7 and 8-8, § B.
The US Naval Handbook (1995) states:
Belligerents are forbidden to bombard a city or town that is undefended and that is open to immediate entry by their own or allied forces. A city or town behind enemy lines is, by definition, neither undefended nor open, and military targets therein may be destroyed or bombarded. 
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:
Belligerents are forbidden to bombard a city or town that is undefended and that is open to immediate entry by their own or allied forces. A city or town behind enemy lines is, by definition, neither undefended nor open, and military targets therein may be destroyed by 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.
Under the US War Crimes Act (1996), violations of Article 25 of the 1907 Hague Regulations are war crimes. 
United States, War Crimes Act, 1996, Section 2441(c)(2).
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 non-defended localities.” 
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.
According to the Report on US Practice, the opinio juris of the United States concerning open towns and undefended areas generally follows the conditions and rules prescribed in Articles 59 and 60 of the 1977 Additional Protocol I. 
Report on US Practice, 1997, Chapter 1.8.