18, Section 2441:
a) Offense.— Whoever, whether inside or outside the United States, commits a war crime, in any of the circumstances described in subsection (b), shall be fined under this title or imprisoned for life or any term of years, or both, and if death results to the victim, shall also be subject to the penalty of death.
(b) Circumstances.— The circumstances referred to in subsection (a) are that the person committing such war crime or the victim of such war crime is a member of the Armed Forces of the United States or a national of the United States (as defined in section 101 of the Immigration and Nationality Act).
(c) Definition.— As used in this section the term “war crime” means any conduct—
(1) defined as a grave breach in any of the international conventions signed at Geneva 12 August 1949, or any protocol to such convention to which the United States is a party;
(2) prohibited by Article 23, 25, 27, or 28 of the Annex to the Hague Convention IV, Respecting the Laws and Customs of War on Land, signed 18 October 1907;
(3) which constitutes a violation of common Article 3 of the international conventions signed at Geneva, 12 August 1949, or any protocol to such convention to which the United States is a party and which deals with non-international armed conflict; or
(4) of a person who, in relation to an armed conflict and contrary to the provisions of the Protocol on Prohibitions or Restrictions on the Use of Mines, Booby-Traps and Other Devices as amended at Geneva on 3 May 1996 (Protocol II as amended on 3 May 1996), when the United States is a party to such Protocol, willfully kills or causes serious injury to civilians.
http://straylight.law.cornell.edu/uscode/html/uscode18/... Because the US is a member of the UN, it must adhere to the Principles of International Law Recognized in the Charter of the Nürnberg Tribunal and in the Judgment of the Tribunal.
Principle VI
The crimes hereinafter set out are punishable as crimes under international law:
(a) Crimes against peace:
(i) Planning, preparation, initiation or waging of a war of aggression or a war in violation of international treaties, agreements or assurances;
(ii) Participation in a common plan or conspiracy for the accomplishment of any of the acts mentioned under (i).
(b) War crimes:
Violations of the laws or customs of war which include, but are not limited to, murder, ill-treatment or deportation to slave-labour or for any other purpose of civilian population of or in occupied territory; murder or ill-treatment of prisoners of war, of persons on the Seas, killing of hostages, plunder of public or private property, wanton destruction of cities, towns, or villages, or devastation not justified by military necessity.
(c) Crimes against humanity:
Murder, extermination, enslavement, deportation and other inhuman acts done against any civilian population, or persecutions on political, racial or religious grounds, when such acts are done or such persecutions are carried on in execution of or in connection with any crime against peace or any war crime.
http://www.un.org/law/ilc/texts/nurnfra.htm