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United States of America
Practice Relating to Rule 29. Medical Transports
Section C. Respect for and protection of hospital ships
The US Air Force Pamphlet (1976) states: “In addition to grave breaches of the Geneva Conventions of 1949, the following acts are representative of situations involving individual criminal responsibility: (1) deliberate attack on … hospital ships”. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 15-3(c)(1).
The US Air Force Commander’s Handbook (1980) provides that ambulances and hospital ships “should not be deliberately attacked, fired upon, or unnecessarily prevented from performing their medical duties”. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-2.
The US Naval Handbook (1995) qualifies “deliberate attack upon hospital ships” as a war crime. 
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), § 6.2.5.
The US Naval Handbook (2007) states that examples of war crimes that could be considered as grave breaches of the 1949 Geneva Conventions include: “Deliberate attacks upon … hospital ships.” 
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.
The US Manual on Detainee Operations (2008) states:
Legal Considerations
a. As a subset of military operations, detainee operations must comply with the law of war during all armed conflicts, however such conflicts are characterized, and in all other military operations …
c. The four Geneva Conventions of 1949 are fully applicable as a matter of international law to all military operations that qualify as international armed conflicts … The principles reflected in these treaties are considered customary international law, binding on all nations during international armed conflict. Although often referred to collectively as the “Geneva Conventions,” the specific treaties are:
(2) [1949] Geneva Convention [II] … This convention … protects hospital ships. 
United States, Manual on Detainee Operations, Joint Chiefs of Staff, 30 May 2008, pp. I-2–I-3.