United States of America
Practice Relating to Rule 23. Location of Military Objectives outside Densely Populated Areas
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. Accordingly, they should endeavor … to avoid locating military objectives within or near densely populated areas. It is incumbent upon states, desiring to make protection of their own civilian population fully effective, to take appropriate measures to segregate and separate their military activities from the civilian population and civilian objects. Substantial military advantages may in fact be acquired by such separation. 
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).

With respect to the result of failure to separate military activities from civilian areas, the Pamphlet states:
The failure of states to segregate and separate their own military activities, and particularly to avoid placing military objectives in or near populated areas and to remove such objects from populated areas, significantly and substantially weakens effective protection for their own population. A party to a conflict which places its own citizens in positions of danger by failing to carry out the separation of military activities from civilian activities necessarily accepts, under international law, the results of otherwise lawful attacks upon valid military objectives in their territory. 
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(b).

In 1966, in the context of the Vietnam War, the US Department of Defense stated:
It is impossible to avoid all damage to civilian areas, especially when the North Vietnamese deliberately emplace their air defense sites, their dispersed POL, their radar and other military facilities in the midst of populated areas, and, indeed, sometimes on the roofs of government buildings. 
United States, Department of Defense, Statement on targeting policy in Vietnam, 26 December 1966, reprinted in Marjorie Whiteman, Digest of International Law, Vol. 10, Department of State Publication 8367, Washington, D.C., 1968, p. 427.

In 1966, in reply to an inquiry from a member of the US House of Representatives asking for a restatement of US policy on targeting in North Vietnam, a US Deputy Assistant Secretary of Defense wrote: “It is impossible to avoid all damage to civilian areas, particularly in view of the concerted effort of the North Vietnamese to emplace anti-aircraft and critical military targets among the civilian population.” 
United States, Letter from Deputy Assistant Secretary of Defense Goulding to US Representative Ogden Reid from New York, 30 December 1966, reprinted in Marjorie Whiteman, Digest of International Law, Vol. 10, Department of State Publication 8367, Washington, D.C., 1968, p. 428.

In 1972, the General Counsel of the US Department of Defense stated:
The principle [contained in paragraph 1(c) of UN General Assembly Resolution 2444 (XXIII) of 1969 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] addresses primarily the Party exercising control over members of the civilian population. This principle recognizes the interdependence of the civilian community with the overall war effort of a modern society. But its application enjoins the party controlling the population to use its best efforts to distinguish or separate its military forces and war making activities from members of the civilian population to the maximum extent feasible so that civilian casualties and damage to civilian objects incidental to attacks on military objectives, will be minimized as much as possible. 
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. 123.

In 1991, in response to an ICRC memorandum on the applicability of IHL in the Gulf region, the US Department of the Army stated:
The obligation of distinguishing combatants and military objectives from civilians and civilian objects is a shared responsibility of the attacker, defender, and the civilian population as such … A defender must exercise reasonable precaution to separate the civilian population and civilian objects from military objectives. 
United States, Message from the Department of the Army to the legal adviser of the US Army forces deployed in the Gulf, 11 January 1991, § 8(E), Report on US Practice, 1997, Chapter 1.7.

In 1991, in a report submitted to the UN Security Council on operations in the Gulf War, the United States denounced Iraq for having “intentionally placed civilians at risk through its behaviour”. The report cited the following examples of such behaviour:
(a) The Iraqi Government moved significant amounts of military weapons and equipment into civilian areas with the deliberate purpose of using innocent civilians and their homes as shields against attacks on legitimate military targets;
(b) Iraqi fighter and bomber aircraft were dispersed into villages near the military airfields where they were parked between civilian houses and even placed immediately adjacent to important archaeological sites and historic treasures;
(c) Coalition aircraft were fired upon by anti-aircraft weapons in residential neighbourhoods in various cities. In Baghdad, anti-aircraft sites were located on hotel roofs;
(d) In one case, military engineering equipment used to traverse rivers, including mobile bridge sections, was located in several villages near an important crossing point. The Iraqis parked each vehicle adjacent to a civilian house. 
United States, Letter dated 5 March 1991 to the President of the UN Security Council, UN Doc. S/22341, 8 March 1991, pp. 2–3.

In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated:
Historically, and from a common sense standpoint, the party controlling the civilian population has the opportunity and responsibility to minimize the risk to the civilian population through the separation of military objects from the civilian population … The defending party must exercise reasonable precautions to separate the civilian population and civilian objects from military objectives, and avoid placing military objectives in the midst of the civilian population. 
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. 625.

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, not the least of which is to take all reasonable measures to separate military objectives from civilian objects and the civilian population. Regrettably, in conflicts such as the Korean and Vietnam Wars, as well as the 1991 Persian Gulf War, the armed forces of the United States have faced opponents who have elected to use their civilian populations and civilian objects to shield military objectives from attack. Notwithstanding such actions, U.S. forces have taken reasonable measures to minimize collateral injury to civilians and damage to civilian objects while conducting their military operations, often at increased risk to U.S. personnel. 
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.

In 1996, the Monitoring Group on the Implementation of the 1996 Israel-Lebanon Ceasefire Understanding, consisting of France, Israel, Lebanon, the Syrian Arab Republic and the United States, pleaded with combatants to respect the precautionary measure of separating military objectives from densely populated areas, re-emphasizing that artillery fired from populated areas endangered civilians. The Monitoring Group also asked combatants to take all necessary precautions during military operations launched from the vicinity of populated areas. 
Monitoring Group on the Implementation of the 1996 Israel-Lebanon Ceasefire Understanding, Fourth and fifth meetings, 22–25 September and 14–18 October 1996.

The Report on US Practice states: “It is the opinio juris of the United States that parties to a conflict should, to the maximum extent feasible, segregate and separate their military activities from the civilian population to protect the latter.” 
Report on US Practice, 1997, Chapter 1.7.