Related Rule
United States of America
Practice Relating to Rule 22. The Principle of Precautions against the Effects of Attacks
Section A. Precautions to protect the civilian population, civilians and civilian objects
The US Air Force Pamphlet (1976) states:
As a corollary to the principle of general civilian immunity, the parties to a conflict should, to the maximum extent feasible, take necessary precautions to protect the civilian population, individual civilians, and civilian objects under their authority against the dangers resulting from 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, § 5-4(a).
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: “The obligation to take reasonable measures to minimize damage to natural resources and cultural property is shared by both an attacker and a defender … The defender has certain responsibilities as well.” 
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. 203.