Related Rule
United States of America
Practice Relating to Rule 13. Area Bombardment
The US Air Force Pamphlet (1976) quotes Article 24(3) of the 1923 Hague Rules of Air Warfare, specifying, however, that “they do not represent existing customary law as a total code”. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-2(c).
The Pamphlet also restates the opinion of a legal scholar concerning target area bombing:
Any legal justification of target-area bombing must be based on two factors. The first must be the fact that the area is so preponderantly used for war industry as to impress that character on the whole of the neighborhood, making it essentially an indivisible whole. The second factor must be that the area is so heavily defended from air attack that the selection of specific targets within the area is impracticable.
In such circumstances, the whole area might be regarded as a defended place from the standpoint of attack from the air, and its status, for that purpose, is assimilated to that of a defended place attacked by land troops. In the latter case, the attacking force may attack the whole of the defended area in order to overcome the defense, and incurs no responsibility for unavoidable damage to civilians and nonmilitary property caused by the seeking-out of military objectives in the bombardment. Legal justification for target-area bombing would appear to rest upon analogous reasoning. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-2(d), referring to Morris Greenspan, The Modern Law of Land Warfare, University of California Press, Berkeley, 1959, p. 336.
The Pamphlet states, however: “In fact, the use of target area bombing in populated areas has always been controversial.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-2(d), footnote 9, referring to James M. Spaight, Air Power and War Rights, Longmans, Green and Co., London/New York/Toronto, Third edition, 1947, p. 272 and Julius Stone, Legal Controls of International Conflict, Garland Publishing, New York/London, 1973, p. 627.
During the CDDH, the US delegation stated that the words “clearly separated” referred
not only to a separation of two or more military objectives, which could be observed or which were usually separated, but to include the element of a significant distance. Moreover, that distance should be at least sufficiently large to permit the individual military objectives to be attacked separately. 
United States, Statement at the CDDH, Official Records, Vol. XIV, CDDH/III/SR.31, 14 March 1975, p. 307, § 50.