The actual definition of war crimes, according to the Charter of the the International Military Tribunal (in the Agreement for the Prosecution and Punishment of the Major War Criminals of the European Axis (London Agreement), August 8, 1945) is:

War Crimes: namely, violations of the laws or customs of war. Such violations include, but not be limited to, murder, ill-treatment or deportation to slave labor or for any other purpose of civilian population of or in occupied territory, murder or ill-treatment of prisoners of war or 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.

http://www1.umn.edu/humanrts/instree/imt1945.htm