Related Rule
United States of America
Practice Relating to Rule 12. Definition of Indiscriminate Attacks
Section C. Attacks whose effects cannot be limited as required by international humanitarian law
The US Air Force Pamphlet (1976) states:
Some weapons, though capable of being directed only at military objectives, may have otherwise uncontrollable effects so as to cause disproportionate civilian injuries or damage. Biological warfare is a universally agreed illustration of such an indiscriminate weapon. Uncontrollable effects, in this context, may include injury to the civilian population of other states as well as injury to an enemy’s civilian population. Uncontrollable refers to effects which escape in time or space from the control of the user as to necessarily create risks to civilian persons or objects excessive in relation to the military advantage anticipated. International law does not require that a weapon’s effects be strictly confined to the military objectives against which it is directed, but it does restrict weapons whose foreseeable effects result in unlawful disproportionate injury to civilians or damage to 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, § 6-3(c).
The US Naval Handbook (2007) states that indiscriminate attacks include “attacks that employ a method or means of combat, the effects of which cannot be limited as required by the law of armed conflict (e.g., bombing an entire large city when the object of attack is a small enemy garrison in the city)”. 
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 1972, the General Counsel of the US Department of Defense stated:
Existing laws of armed conflict do not prohibit the use of weapons whose destructive force cannot be limited to a specific military objective. The use of such weapons is not proscribed when their use is necessarily required against a military target of sufficient importance to outweigh inevitable, but regrettable, incidental casualties to civilians and destruction of civilian objects … I would like to reiterate that it is recognized by all states that they may not lawfully use their weapons against civilian population[s] or civilians as such, but there is no rule of international law that restrains them from using weapons against enemy armed forces or military targets. The correct rule of international law which has applied in the past and continued to apply to the conduct of our military operations in Southeast Asia is that “the loss of life and damage to property must not be out of proportion to the military advantage to be gained”. 
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. 124.
According to the Report on US Practice, at the 1974 Lucerne Conference of Government Experts on Weapons which may Cause Unnecessary Suffering or have Indiscriminate Effects, the United States rejected any effort to label weapons indiscriminate merely because they were likely to affect civilians as well as military objectives. The correct rule was that the law of war prohibits attacks which entail a high risk of civilian casualties clearly disproportionate to the military advantage sought. 
Report on US Practice, 1997, Chapter 1.4; Statement of 25 September 1974 at the Conference of Government Experts on Weapons which may Cause Unnecessary Suffering or have Indiscriminate Effects, Lucerne, 24 September–18 October 1974, reprinted in Arthur W. Rovine, Digest of United States Practice in International Law, 1974, Department of State Publication 8809, Washington, D.C., 1975, p. 713.
Course material from the US Army War College states:
The Law of War does not ban the use of weapons when their effects cannot be strictly confined to the specific military objective. But this rule is true only so long as the rule of proportionality is not violated. However, a weapon which is incapable of being controlled, and thus will cause incidental damage without any reasonable likelihood of gaining a military advantage, is illegal. 
US Army War College Selected Readings, Advanced Course, Law for the Joint Warfighter, Vol. II, Second edition, 1989, p. 170.
In 1992, a legal review by the US Department of the Air Force of the legality of extended range anti-armour munition stated:
International law also forbids the use of weapons or means of warfare which are “indiscriminate.” A weapon is indiscriminate if it cannot be directed at a military objective or if, under the circumstances, it produces excessive civilian casualties in relation to the concrete and direct military advantage anticipated. 
United States, Department of the Air Force, The Judge Advocate General, Legal Review: Extended Range Antiarmor Munition (ERAM), 16 April 1992, § 4.
Prior to the adoption of UN General Assembly Resolution 47/37 in 1992 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”, which provided:
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 1993, in its report to Congress on the protection of natural and cultural resources during times of war, the US Department of Defense stated:
Finally, with the poor track record of compliance with the law of war by some nations, the United States has a responsibility to protect against threats that may inflict serious collateral damage to our own interests and allies. These threats can arise from any nation that does not have the capability or desire to respect the law of war. One example is Iraq’s indiscriminate use of SCUDs during the Iran–Iraq War and the Gulf War. These highly inaccurate theater ballistic missiles can cause extensive collateral damage well out of proportion to military results. 
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.