Related Rule
Practice Relating to Rule 70. Weapons of a Nature to Cause Superfluous Injury or Unnecessary Suffering
In its response to submissions from other States to the ICJ in the Nuclear Weapons (WHO) case in 1995, Nauru stated: “It is also a violation of customary international law to use weapons that cause unnecessary suffering.” 
Nauru, Response to submissions of other States submitted to the ICJ, Nuclear Weapons (WHO) case, 15 June 1995, Part 1, p. 11.