From the United Nations website.
For the purpose of this Statute, ‘war crimes’ means:
Grave breaches of the Geneva Conventions of 12 August 1949, namely, any of the following acts against persons or property protected under the provisions of the relevant Geneva Convention:
1)Wilful killing
2)Torture or inhuman treatment, including biological experiments;
3)Wilfully causing great suffering, or serious injury to body or health;
4)Extensive destruction and appropriation of property, not justified by military necessity and carried out unlawfully and wantonly;
5)Compelling a prisoner of war or other protected person to serve in the forces of a hostile Power;
6)Wilfully depriving a prisoner of war or other protected person of the rights of fair and regular trial;
7)Unlawful deportation or transfer or unlawful confinement;
8)Taking of hostages.
See points 3,4 and arguably 7.