Related Rule
United States of America
Practice Relating to Rule 20. Advance Warning
The US Field Manual (1956) states: “The officer in command of an attacking force must, before commencing a bombardment, except in cases of assault, do all in his power to warn the authorities.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 43.
The US Air Force Pamphlet (1976) states: “Effective advance warning shall be given of attacks which may affect the civilian population unless circumstances do not permit.” 
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)(iii).
The Pamphlet specifies that:
The practice of states recognizes that warnings need not always be given. General warnings are more frequently given than specific warnings, lest the attacking force or the success of its mission be jeopardized. Warnings are relevant to the protection of the civilian population and need not be given when they are unlikely to be affected by the attack. 
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)(2)(d).
The US Naval Handbook (1995) states:
When circumstances permit, advance warning should be given of attacks that might endanger noncombatants in the vicinity. Such warnings are not required, however, if mission accomplishment requires the element of surprise or the security of the attacking forces would be otherwise compromised. 
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), § 11.2; see also § 8.5.2.
The Handbook specifies that: “Warnings may be general rather than specific lest the bombarding force or the success of its mission be placed in jeopardy.” 
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), § 8.5.2.
The US Naval Handbook (2007) states:
Where the military situation permits, commanders should make every reasonable effort to warn the civilian population located in close proximity to a military objective targeted for bombardment. Warnings may be general rather than specific lest the bombarding force or the success of its mission be placed in jeopardy. 
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, § 8.9.2.
During the Second World War, before the atomic bomb was dropped on Hiroshima, the US reportedly warned Japanese authorities that certain towns could be heavily bombed and that civilians should be evacuated. Similar warnings were reportedly issued in the European theatre of war. 
Leslie C. Green, The Contemporary Law of Armed Conflict, Melland Schill Monographs in International Law, Manchester University Press, Manchester, 1993, p. 148.
It is reported that, during the Korean War, US forces planned that “several days prior to the attack planes would drop leaflets over Pyongyang warning civilians to stay away from military installations of any kind”. 
Robert F. Futrell, The United States Air Force in Korea 1950–1953, Office of Air Force History, US Air Force, Washington, D.C., Revised edition, 1983, p. 516; see also p. 518.
In 1987, the Deputy Legal Adviser of the US Department of State stated that the United States supported the requirement that “effective advance warning be given of attacks which may affect the civilian population, unless circumstances do not permit”. 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 427.
In 1991, in response to an ICRC memorandum on the applicability of IHL in the Gulf region, the US Department of the Army stated:
A warning need not be specific; it may be a blanket warning, delivered by leaflets and/or radio, advising the civilian population of an enemy nation to avoid remaining in proximity to military objectives. The “unless circumstances do not permit” recognizes the importance of the element of surprise. Where surprise is important to mission accomplishment and allowable risk to friendly forces, a warning is not required. 
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(I), Report on US Practice, 1997, Chapter 1.6.
In 1995, an opinion of a US army legal adviser on the legality of silencers/suppressors stated:
There is no law of war requirement that a combatant must be “warned” before he or she is subject to the application of lawful, lethal force … [The opinion then cites Article 26 of the 1907 Hague Regulations and refers to Article 6 of the 1907 Hague Convention (IX).] Article 57, paragraph 2(c) of Protocol I Additional to the 1949 Geneva Conventions of 8 June 1977 contains a more relaxed but similar requirement, updating the two 1907 provisions while reconciling the slight difference between them. Although not a party to this treaty, the United States regards this provision as a re-codification of customary international law. The warning requirement cited above was for the purpose of enabling the civilian population to take appropriate steps to protect themselves from the collateral effects of attack of military objectives, or otherwise from the effects of war; it is not an obligation to warn combatants of their imminent attack. The exception to the warning requirement, relieving a commander from the obligation in cases of assault (stated more generally as “unless circumstances do not permit” in the 1977 Additional Protocol I) recognizes the legitimate use of the fundamental military element of surprise in the attack of enemy military forces in order to reduce risk to the attacking force and to increase its chance for successful accomplishment of its mission. 
United States, Legality of Silencers/Suppressors, Memorandum for Chief, Operational Law, Headquarters, 101st Airborne Division (Air Assault), 9 June 1995, pp. 6–7.
The Report on US Practice states:
In U.S. practice, bombardment warnings have often been general in their terms, e.g. advising civilians to avoid war-supporting industries, in order not to alert the air defense forces of an impending attack on a specific target. Such was the case in the Korean War. 
Report on US Practice, 1997, Chapter 1.6.