Related Rule
United States of America
Practice Relating to Rule 16. Target Verification
The US Rules of Engagement for the Vietnam War (1971) stated:
All possible means will be employed to limit the risk to the lives and property of friendly forces and civilians. In this respect, a target must be clearly identified as hostile prior to making a decision to place fire on it. 
United States, Rules of Engagement for the Employment of Firepower in the Republic of Viet-Nam, US Military Assistance Command Viet-Nam, Directive No. 525-13, May 1971, unclassified contents reprinted in Eleanor C. McDowell, Digest of United States Practice in International Law, 1975, US Department of State Publication 8865, Washington, D.C., 1976, pp. 814–815, § 6(a).
The US Air Force Pamphlet (1976) states:
Those who plan or decide upon an attack must do everything feasible to verify that the objectives to be attacked are neither civilians nor civilian objects and not subject to special protection but are military objectives and that it is permissible to attack them. 
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)(b)(i)(a).
The US Naval Handbook (1995) states: “All reasonable precautions must be taken to ensure that only military objectives are targeted.” 
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 that “all reasonable precautions must be taken to ensure that only military objectives are targeted”. 
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.
The Report on US Practice refers to an instance recorded during the Vietnam War in the early 1970s when a possible storage facility for air defence missiles, which would normally have been a high-priority target, was removed from the target list because it was “in a heavily populated area on the edge of Hanoi and the intelligence which indicated that it might be a storage facility was somewhat speculative”. 
Report on US Practice, 1997, Chapter 1.6, referring to James R. McCarthy and George B. Allison, Linebacker II: A View From the Rock, US Air Force Southeast Asia Monograph Series, Volume VI, Monograph 8, 1979, pp. 97–98.