Prohibited conduct:
“(D) Murder.—
The act of a person who intentionally kills, or conspires or attempts to kill, or kills whether intentionally or unintentionally in the course of committing any other offense under this subsection, one or more persons taking no active part in the hostilities, including those placed out of combat by sickness, wounds, detention, or any other cause”
-According to Additional Protocol I to the Geneva Conventions, combatants are:
the armed forces of a party to a conflict, and also
groups and units that are under a command responsible to that party for the conduct of its subordinates, even if that party is answerable to a government or an authority not recognized by an adverse party. Such armed forces shall be subject to an internal disciplinary system, which, inter alia, shall enforce compliance with the rules of international law applicable in armed conflict
18 U.S. Code § 2441 - War crimes
Prohibited conduct: “(D) Murder.— The act of a person who intentionally kills, or conspires or attempts to kill, or kills whether intentionally or unintentionally in the course of committing any other offense under this subsection, one or more persons taking no active part in the hostilities, including those placed out of combat by sickness, wounds, detention, or any other cause”
https://www.law.cornell.edu/uscode/text/18/2441