Related Rule
United States of America
Practice Relating to Rule 15. The Principle of Precautions in Attack
Section A. Constant care to spare the civilian population, civilians and civilian objects
The US Air Force Pamphlet (1976) states: “In conducting military operations, constant care must be taken to spare the civilian population, civilians, and civilian objects.” 
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(c)(1)(a).
The US Naval Handbook (1995) states: “All reasonable precautions must be taken to ensure that only military objectives are targeted so that civilians and civilian objects are spared as much as possible from the ravages of war.” 
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), § 8.1.
The US Naval Handbook (2007) states: “The law of targeting, therefore, requires that all reasonable precautions must be taken to ensure that only military objectives are targeted so that noncombatants, civilians, and civilian objects are spared as much as possible from the ravages of war.” 
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, § 8.1.
In 1972, the General Counsel of the US Department of Defense stated that the United States regarded the principle contained in UN General Assembly Resolution 2444 (XXIII) of 1968 that “a distinction must be made at all times between persons taking part in the hostilities and members of the civilian population to the effect that the civilians be spared as much as possible … as declaratory of existing customary international law”. 
United States, Letter from J. Fred Buzhardt, General Counsel of the Department of Defense, to Senator Edward Kennedy, Chairman of the Subcommittee on Refugees of the Committee on the Judiciary, 22 September 1972, AJIL, Vol. 67, 1973, p. 122.
According to the Report on US Practice, it is the opinio juris of the United States that a “distinction must be made between persons taking part in the hostilities and members of the civilian population to the effect that the civilians be spared as much as possible”. 
Report on US Practice, 1997, Chapter 1.4.