United States of America
Practice Relating to the Use of Prohibited Weapons
The US Field Manual (1956) provides: “In addition to the ‘grave breaches’ of the Geneva Conventions of 1949, the following acts are representative of violations of the law of war (‘war crimes’): … making use of … forbidden arms or ammunition.” 
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, § 504(a).
The US Instructor’s Guide (1985) provides that “in addition to the grave breaches of the Geneva Conventions, the following acts are further examples of war crimes: using … forbidden arms or ammunition”. 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 13.
The US Naval Handbook (1995) states: “The following acts are representative war crimes: … employing forbidden arms or ammunition.” 
United States, Naval 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), § 6.2.5(10).
The US Naval Handbook (2007) states that “[e]mploying forbidden arms or ammunition” is an example of an act that could be considered a war crime. 
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, § 6.2.6.