Related Rule
United States of America
Practice Relating to Rule 12. Definition of Indiscriminate Attacks
Section A. Attacks which are not directed at a specific military objective
The US Air Force Pamphlet (1976) states:
The extent to which a weapon discriminates between military objectives and protected persons and objects depends usually on the manner in which the weapon is employed rather than on the design qualities of the weapon itself. Where a weapon is designed so that it can be used against military objectives, its employment in a different manner, such as against the civilian population, does not make the weapon itself unlawful. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 6-3(c).
The US Naval Handbook (1995) states: “Any weapon may be set to an unlawful purpose when it is directed against noncombatants and other protected persons and objects.” 
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), § 9.1.
The US Naval Handbook (2007) states that indiscriminate attacks include “attacks that are not directed at a specific military objective (e.g., Iraqi SCUD missile attacks on Israeli and Saudi cities during the Persian Gulf 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, § 5.3.2.
In 1991, in a report submitted to the UN Security Council on operations in the Gulf War, the United States denounced the continued indiscriminate launching of surface-to-surface missiles at civilian targets. 
United States, Letter dated 5 March 1991 to the President of the UN Security Council, UN Doc. S/22341, 8 March 1991, p. 1.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense accused Iraq of “indiscriminate Scud missile attacks”. 
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. 635.
Prior to the adoption in 1992 of UN General Assembly Resolution 47/37 on the protection of the environment in times of armed conflict, Jordan and the United States submitted a memorandum to the Sixth Committee of the UN General Assembly entitled “International Law Providing Protection to the Environment in Times of Armed Conflict”. The memorandum stated:
It is a war crime to employ acts of violence not directed at specific military objectives, to employ a method or means of combat which cannot be directed at a specific military objective, or to employ a means or method of combat the effects of which cannot be limited as required by the law of armed conflict. 
Jordan and United States, International Law Providing Protection to the Environment in Times of Armed Conflict, annexed to Letter dated 28 September 1992 to the Chairman of the Sixth Committee of the UN General Assembly, UN Doc. A/C.6/47/3, 28 September 1992, § 1(g).
In its written statement submitted to the ICJ in the Nuclear Weapons (WHO) case in 1994, the United States stated: “It is unlawful to conduct any indiscriminate attack, including those employing weapons that are not … directed at a military objective.” 
United States, Written statement submitted to the ICJ, Nuclear Weapons (WHO) case, 10 June 1994, p. 27.
In submitting the 1996 Amended Protocol II to the Convention on Certain Conventional Weapons to Congress for advice and consent to ratification, the US President stated that the prohibition of indiscriminate use of mines, booby-traps and other devices as defined in Article 3(8)(a) of the Protocol “is already a feature of customary international law that is applicable to all weapons”. 
United States, Message from the US President transmitting Protocols II, III and IV to the Convention on Certain Conventional Weapons to the Senate for consent to ratification, Treaty Doc. 105-1, Washington, D.C., 7 January 1997, Tab (A), p. 13.
According to the Report on US Practice, it is the opinio juris of the United States that indiscriminate attacks include attacks which are not directed at a military objective. 
Report on US Practice, 1997, Chapter 1.4.