Related Rule
United States of America
Practice Relating to Rule 10. Civilian Objects’ Loss of Protection from Attack
Section A. Civilian objects used for military purposes
The US Air Force Pamphlet (1976) states: “The inherent nature of the object is not controlling since even a traditionally civilian object, such as a civilian house, can be a military objective when it is occupied and used by military forces during an armed engagement.” 
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(b)(2).
The US Rules of Engagement for Operation Desert Storm (1991) gives the following instruction:
Do not fire into civilian populated areas or buildings which are not defended or being used for military purposes … Do not attack traditional civilian objects, such as houses, unless they are being used by the enemy for military purposes and neutralization assists in mission accomplishment. 
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 and G.
The US Naval Handbook (2007) states that “misuse of protected places and objects for military purposes renders them subject to legitimate attack during the period of misuse”. 
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, § 8.10.2.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated: “Civilian objects are protected from direct, intentional attack unless they are used for military purposes, such as shielding military objects from attack.” 
United States, Department of Defense, Final Report to Congress on the Conduct of the Persian Gulf War, 10 April 1992, Appendix O, The Role of the Law of War, ILM, Vol. 31, 1992, p. 622.
In 1993, in its report to Congress on the protection of natural and cultural resources during times of war, the US Department of Defense stated: “Cultural property, civilian objects, and natural resources are protected from intentional attack so long as they are not utilized for military purposes.” 
United States, Department of Defense, Report to Congress on International Policies and Procedures Regarding the Protection of Natural and Cultural Resources During Times of War, 19 January 1993, p. 202.