Related Rule
Practice Relating to Rule 14. Proportionality in Attack
Ukraine’s IHL Manual (2004) states:
Principle of proportionality means that the parties to the conflict during hostilities must refrain from damaging civilian objects and causing losses of human population that would be excessive in order to obtain sufficient advantage in strength over the enemy.
The principle of proportionality must not be used to justify unlimited damage or attacks against civilian persons and objects. 
Ukraine, Manual on the Application of IHL Rules, Ministry of Defence, 11 September 2004, § 2.1.8.